Good answers from tbrice23 and Greyfox. Think of it like this: recoil is energy, and energy wants to take the path of least resistance. If you aren't directly squared up behind the rifle you are creating a pivot point (with your shoulder and buttstock) for that energy to divert the muzzle at an angle and not straight back. There are ways to mitigate this in other firing positions but prone is a good place to start. I like to line/square up behind the rifle and load the bipod (started using spiked bipod feet and it has help quite a bit). I personally get fairly aggressive with it, while bending my non-trigger fingers on my firing hand @ a 90'ish degree angle and pulling straight back on the grip/palm swell (or whatever). Your trigger finger wants to do as the others are doing, a sympathetic movement so to speak. Why not let it while engaging the trigger.
Calling the shot is as described, knowing exactly where your point of aim (POA) is when the shot is broke. We practiced this with Sniper teams. When the shot breaks the shooter will announce where the POA was on the target so that the spotter could give an accurate reading/correction and not just base it off intended POA.
As for knowing when the trigger will release the sear/firing pin and send a round down range, you need to know this when you have to qualify on movers out to 600 (now at night). Most of the guys I know would "ambush" the target, in other words pick a spot in front of the target and firing at a certain lead ahead of the target (when it reaches a certain point on your horizontal reticle). You have to "make" the rifle go off exactly when you want it to, no guessing about it...those targets are fairly thin.
