What is Low Pressure (LP)?
Low Pressure operation is a relative term, but commonly refers to operating at lower than what is considered "normal" pressure.  Typically, Co2 tanks output anywhere from 750 psi to 1000 psi and above on a warm day, and most preset High Pressure Air (HPA) systems deliver around 750 psi.  Unregulated blow-back semi-auto paintball guns are generally designed to operate in these "normal" pressure ranges.

How do LP systems work?
Acceleration of the paintball requires a combination of both gas pressure and volume. Lowering the operating pressure means more reliance on volume to accelerate the ball to a given speed. On most blow-back guns, this means the modification of the valve system to allow a larger volume of gas to impact the ball, and the addition of a regulator, to control the operating pressure.

What are the advantages of LP operation?
Although there are a large number of unproved theories concerning accuracy and range, there are several actual, quantifiable effects from converting a blow-back semi-automatic paintball gun to LP operation. Whether or not these are considered real advantages depends on what you wish to achieve.
The most obvious effect of LP operations is reduced noise signature. LP guns use less pressure to propel the ball, and a lower pressure means a lower audible "pop" sound when gas is released from the valve.
Due to the nature of Co2, users of that propellant will see biggest advantages from LP, such as improved cold weather performance.  Co2 pressure fluctuates with temperature, and in cold weather, tank output pressure can drop below 600 psi. A gun requiring 750 psi to operate will simply not be able to function due to a lack of necessary pressure. However, a gun requiring only 350 psi to operate will continue to function. Along the same lines, shoot-down is less likely to occur. Shoot-down is a phenomenon which occurs when the valve system is starved for gas under rapid firing conditions.  This is often the result of the Co2 tank chilling (and pressure dropping) from the rapid use of gas. A gun operating at LP is less likely to feel the effect of the tank pressure dropping.
LP operation also results in less "blow-back" of excess gas up the feed tube of the gun. This "blow-back" can impede the steady feeding of balls during rapid firing.
 

What are the Disadvantages of LP operation?
A properly balanced LP set-up has no obvious disadvantages, outside of the initial investment cost to make the conversion. Although some people may experience reliability issues, or a substantial drop in efficiency following the conversion process, these problems are the result of an improper set-up.

Will LP operations make my gun more efficient?
More efficient operation is a side effect of a properly balanced LP set-up. If the right pressure vs. volume level is achieved, it can result in a substantial increase in efficiency when compared to most "normal" pressure systems, but at the same time, an unbalanced system can result in the exact opposite. Keep in mind that ball acceleration requires a combination of both gas pressure and volume. Most blow-back guns, being designed to operate on normal Co2 tank pressures, rely heavily on pressure to get the job done. By lowering the operating pressure, you are in effect increasing the system's reliance on volume. Lowering the pressure somewhat can increase efficiency, as you balance the system's needs for both pressure and volume, without excessive reliance on one or the other. Lowering the pressure somewhat also allows your gun to use more of the available gas in the tank, beyond the point where normal operations would cease because of a drop in pressure. However, lowering the pressure too much can cause an excessive reliance on volume, making the gun inefficient. When converting a gun to LP operation, it is important to remember that your overall goal should not concern achieving the lowest possible operating pressure, but instead, the most efficient operating pressure for that particular gun.
(Note: As an example, our personal experience shows the typical best results are achieved when blow-backs are tuned to operate in the neighborhood of 350 to 400 psi. This results in + 30% increase in efficiency when compared to normal pressure operation, depending on the type of gun and modifications made. Overall, going very far  below 300 psi appears to result in a serious deficiency in efficiency, as well as more reliability problems.)

What is needed to make a gun operate at LP?
What it takes to convert a blow-back semi-auto to LP operation depends on the specifics of the gun. Some blow-backs are easier to convert than others, having drop-in kits readily available, while other blow-backs, due to their design, are very difficult to convert.
In all cases, an adjustable regulator is needed to control the pressure coming from the gas source. This can be an aftermarket part, an accessory included with the gun, or an adjustable regulator attached to the gas source itself. For Co2 users, it's also a good idea to install an anti-siphon system of some kind, to keep liquid Co2 out of the system.
In most cases modifications will have to be made to the gun to increase the volume of pressurized gas standing ready to be used, and to improve the ability of that gas to flow through the valve system.  In essence, the modifications concern moving a high volume of gas as efficiently as possible. For guns such as a Spyder, typically the modifications will include the use of a volume chamber extension (a.k.a. LP chamber) and a high-flow valve and valve pin which have been ported and cut to allow higher flow rates.

Can all blow-backs be converted to LP?
With enough work, any blow-black can be converted to operate at lower than normal pressure. However, the amount of work it would require to convert some guns makes the exercise impractical. For example, because the placement of the valve is in the middle of the gun, In-line blow-backs (such as Tippmann guns) are not nearly as easy to modify for LP operations. Tippmann recently introduced an LP kit for the Model 98, which combats the valve accessibility problem by placing the LP chamber directly below a modified valve. For other in-line models, LP modification is largely reserved to those who are creative, resourceful and good with tools.
Most modern two-tube (a.k.a. "stacked") blow-backs such as the Spyder or Piranha, benefit from a wide availability of aftermarket parts, and a design which lends itself to easy modification. Some of the "higher end" versions of these guns even come with parts like the LP chamber already in place, however, it should be noted that the implementation is often "gimmicky", obviously aimed more toward cosmetic improvement rather than performance. For instance, modern Spyders with "compact" style bodies have an LP chamber included, but the connection to the inside of the valve chamber is through a tiny pin hole* in the vertical ASA, which completely ruins the idea of storing a large volume of gas and moving it efficiently through the valve system. To take advantage of the LP chamber, the small hole would have to be opened (a process which could damage the ASA if performed improperly) or an aftermarket ASA with higher flow capabilities (about $20) would need to be installed. (*For a picture see the Entry Level Gun Comparison.)

What are the most typical problems with blow-back LP operation?
The most typical problems experienced with converting a blow-back to LP operation concern balancing the system's needs for gas to propel the ball, and gas to re-cock the gun - two actions which occur simultaneously.  An improperly set-up gun may produce adequate velocity at a low pressure, yet refuse to re-cock, or the exact opposite may be true where the gun will re-cock, yet not be able to achieve a usable velocity. Although this can also be a problem with normal pressure operations, the effect is often more pronounced during LP operations. Sometimes a simple valve or main spring change to increase the overall amount of gas into the valve system fails to solve the problem.  During LP operations, several factors which are not as critical under normal operating pressures play a larger role in the equation, such as the amount (volume) of available gas stored directly behind the valve and how that available gas is directed through the valve for both duties.
For example, in a Spyder, the shape of the valve pin, and size of the upper opening in the valve can have an impact on how much gas flows upward to impact the ball, and how much flows toward the rear of the gun for re-cocking the action. There is only so much gas available inside the valve chamber area, so tuning becomes a case of robbing Peter to pay Paul. When modifications are made to increase the amount of gas headed in one direction, it is decreased in the other. If a balance cannot be struck which directs enough gas for both propelling the ball and re-cocking the gun, then there are two options - increase the amount (volume) of available gas (increase the size of the valve chamber, hence valve chamber extensions) or increase the pressure of the available gas.
 

Is converting to LP worth it?
Aside from the financial investment, the conversion process to LP operation may take some extensive fine tuning to strike a proper balance. Whether or not converting to LP is worth the time or the money depends on several factors, not the least of which is a consideration of what you are attempting to achieve.  If your performance goals are focused on the "real" advantages LP operation offers, and the financial aspect is not a problem, then there's your answer.  On the other hand, if you are converting to LP simply to follow the latest trend, or because you made the mistake of believing in unproved theories, you may be disappointed.