the reason they need ur gun is bc they take the valve and screw it into your gun, that way the Anti-syphon will work by not sucking in liquid Co2
Correct Anti-syphon
Red = anti-syphon tube Blue = CO2 liqid
As you can see, When installed correctly (you must bring your gun in so they can fit the valve to your gun) the Tube will allow the vlave to only suck up the Gas CO2 inside the tank, and no Liquid gets in the tube
Incorrect Anti-Syphon
As you can see, If you didn't bring ur gun to them when they install it, The tube might be fitten wrong, and will become a syphon tube (only sucks up the liquid) which you totaly messed up your shiznite
Hope this helps... took me like 15 min draw those pictures w/ my ghetto Paint skillz