there will be no "bounce". The piston will come to rest gently against the end of the compression tube, slowed by the compression of air ahead of it. The pellet provides just enough resistance to allow that to happen. For example, we know we should never dry fire a springer because of the possibility of damage to the internals -ie. no resistance, and the piston will slam against the front of the compression tube.
An HDD is a device that helps to preserve air or gas in a reservoir, by preventing the hammer from bouncing back from the valve stem on its initial strike, recompressing the hammer spring, and then opening the valve a second time. Even though the duration of the second (or third) strike of the valve stem is very short compared to the initial opening, some gas is still wasted. There is no need for that technology in a springer. A springer makes its own gas. As do we all.....
Maybe someone else can do a better job of explaining this.
