The arm/hand across the chest....thing? Many sources. Many reasons.
In the last five years I have seen hundreds, maybe over a thousand shooters interact with all kinds of simulated ammo...paintball...sims...pellets...yes, even rubber bands! I have staged all kinds of scenarios. And in these scenarios, I have seen range vets suddenly shoot one-handed and novices shoot two-handed! All from situational stress. Seems like decades of training go out the window with a liittle interactive fire.
In the big picture I just can't officially declare one was wrong and one was right. People just do what they do sometimes despite the training. I think it is important to see what each person does under their own sense of stress and use that as a platform to build on. Such is impossible on a grand scale of running groups, academies and militaries through schedules. That has never been good for the individual.
I myself have mucho training hours one-hand shooting from the late 60s and early 70s. Taught to me by old FBI agents and even old Army guys. I throw that pistol out, center-line and under my nose with a tight or extended arm and shoot. It is still the most comfortable way for me. I qualified expert many times like this, even with a snub-nose .38 on the 50-round courses! And, old methodology or not...I will kill you this way. You may kill me right back your way! As Mike Gillette like to say, "Its all different shades of terrible."
If you make me shoot at something 15 feet or so away, I am compelled to grab that gun with two hands, find that sight and do the usual thing. Hey, the oldtimers taught that too!
But two-handed grips:
-steady the weapon on target
-reduce recoil
-serve as weapon retention
All very noble and smart. It is just hard to get some people to do this, even some of the heavily trained, when bullets fly! I do wish you could always use two hands and find thise sights! Get that Zen breath and squeeze....but...
I always joke around and call one-hand shooting "God-shooting," as in it seems so natural and instinctive. This seems to take over years of two-hand range shooting in some people under combat stress. It cannot be ignored!
So, in the sciences of one-hand shooting, the "other" arm was primarily up to deflect rounds from your chest. The flight paths of bullets can be fickle. If you take one to the hand? It may travel down your arm instead entering your chest or throat. Your free-hand elbow is down a bit and may cover that damn opening in the armpit of your bullet-proof vest, or your heart if you are vest-less.
One-hand? two-hand? Structured range stances? When we do mutilple opponent, close-quarter, sims drils (Three-way-Splitters I call them) the shooter is running around like a football player shooting and fending off mulitple attackers charging with sticks, knives and guns from many angles, before and after they are shot (see Training Mission Six DVD set). This shooter is absolutely STANCELESS and doing the best he can to stay alive.
Oops gotta run! But you get most of the idea...
Hock
