Here is a very good explaination of it:
maybe let me try to put it another way. once you rack the slide the striker is cocked to a point that if, all of the sudden in a millisecond, the striker block and the sear disappeared, the striker would spring forward and ignite the primer and fire the gun. Gun condition codes don't seem to apply the same to striker guns because all the codes are set up for DA\SA guns with a hammer, but you can kind of think of the M&P system like condition 1/2... mag loaded, round chambered, Striker(hammer) cocked, safety off. except that we now have striker block safeties and trigger shoe safeties which are automatic as you pull the trigger. Somewhere between condition zero and condition one.
that 2% of movement, like Ray noted is basically just there to add some friction to make the trigger "safer".
If you look at apex sears, they actually remove material from the little nub that engages the striker to lessen that friction Ray mentioned. it also serves to lighten the trigger pull and make it more crisp so it doesn't have that sort of slightly round feel the stock triggers have. that's why the sell the duty/carry kits with heavier than stock springs.
anyway, hopefully this all helps you understand the system a little better. I've harped on it enough so I'm gonna bow out of this conversation and get on road.