I'm no gunsmith nor do I play one on TV but I believe most striker fired guns (M&P, Glock, Xd, etc.) use the rearward movement of the slide to 'cock' or tension the striker. Some versions completely load the striker and other systems partially load the striker. The trigger is used to either complete the cocking of the striker and then release that stored energy or to simply release the energy if the system has been fully loaded by the slide's movement. In either case the slide must move rearward (by being fired or manually racking the slide) to 'cock' the striker mechanism. If you google striker vs. hammer fired you'll find lots of info and some cool animations. I'm sure it has been explained better somewhere on the forum as well.