It is a pressure balancing valve that balances the pressure at both ends of the chamber.
traditionally you have a cupseal being held in place by a spring and air pressure forcing it to stay closed.
One style of balanced valves is drilling a small hole in the cupseal and making a second area behind the transfer portion of the valve that holds air to assist in pushing the valve open, these are called air assist valves, lucky made them for a bunch of different guns as well as many other companies.
The other major design style is what im calling a newton valve setup because that is what evolve called it when they made it for the a4 series of angel. It has been done before and smart parts has done it again for there new imp. This style extends the cupseal to be sealed at the valve and again at the other end of the dump chamber, with the spring outside of the dump chamber. This makes the air pressure act similar on both sides of the cupseal and negates its effect so essentially only the spring is keeping the valve closed. This allows you more control over the system in this configuration, and added benefits are it makes opening the valve very easy so a low moving mass, dwell, and lpr pressure is needed meaning a smoother more efficient gun. Not to mention there is also no free space in front of the valve and the lack of a spring makes the air have less turbulence and a more linear path through the chamber.
same valve setup in an angel. That angel has 20 grams of moving mass with the delrin bolt and hammer smooth as butter, has an lpr pressure of 40 psi, dwell of 9,and gets a case and a pod off a 68/45. It is a proven concept and a great valve design when done correctly.