As soon as the wheel is released by the sear, the longer arm of the mainspring pulls the chain engaged in it. The other end of the chain being fixed to the cam on the wheel shaft, the latter rotates at high speed, whilst the rotating cam pushes forward the arm to which the pan cover is attached, thus causing the pan cover to slide forward towards the muzzle of the piece, and the pyrites to fall (under tension of the dog spring) on to the now rotating wheel. That is the second purpose of a sliding pan cover: were the pyrites to engage a stationary wheel, it would almost certainly jam the mechanism: but the built-in delay allows the pyrites to slip off the sliding pan-cover on to an already rotating wheel. A more modern development has been the use of a ball bearing between the wheel and the sear. This design allows a smoother and lighter trigger pull, requiring less force to operate.