This a pretty good explanation on what it does, basically hammer hit the pin once >>> got this on one of them 2240 mod sites..........
"made an HDD (hammer debounce device) out of the polyethylene tubing, and it worked ok, but the material is too soft to last. I'm going to make one out of some harder plastic at some point. It gave a lot more shots, since less CO2 gets wasted when the hammer isn't bouncing into the valve multiple times per trigger pull. This allows the hammer to fly forward without any drag, but as soon as it starts snapping backards the HDD expands against the walls of the bolt bore and brakes it. This stops the hammer from opening the valve multiple times as it bounces back and forth between the hammer spring and valve spring tension, which obviously just wastes gas. It fits over the hammer's cocking pin, with the slot/opening facing backards. The opening/slot is what makes the device expand, the hammer snapping backwards makes the pin attempt to back out of that opening/slot and makes the body expand in that bolt bore, creating friction to slow the hammer. Forward motion isn't restricted though, so your shot velocity remains unaffected. You just get way more shots per fill since it isn't re-opening the valve after the pellet's already gone."