I'll skip all the technical stuff and use what I hope are terms most folks can understand.
Muscles tend to work together in groups, and some groups are stronger than others.
Still with me?
When it comes to upper body muscles, the strongest on the front are the pectorals, i.e. the chest. Basically, these move the upper arm toward the centerline of your body. Sit on a pec-deck machine, arms held up like you are doing a double biceps shot and then move the elbows to the center against the weight, that's the isolation move. Bench press uses these muscles, though not in the full range of motion.
The antagonists -- the muscles that move the arm the other way, are mostly rear deltoid -- back of shoulder -- with some rotator cuff and upper back stabilizers.
In most men, the chest muscles are stronger than the arms or shoulders. Easy to see: How much weight can you bench versus how much you use doing a bent-over deltoid fly?
So, since strong muscle beats weak muscle, let's postulate a situation in which your friendly neighborhood mugger comes at you with a knife in an overhead stab. You have a bad leg and you can't run, but you also have a knife of your own, and through a miracle of timing, as he attacks, you manage to grab his knife wrist. Unfortunately, he also grabs your knife wrist.
For the sake of the illustration, you are both right-handed and holding your blades in those hands.
Can you picture this? Two players facing each other, each holding on to the other's knife wrist.
If you are of a size and similar strength, stabbing the other guy to the body is going to be difficult, and it's easy to see why: You are both using the same muscles -- chest, arms, shoulders, and maybe some abs and other stabilizers. And since you can resist more force this way than you can usually generate, that is negative reps can use more weight than positive ones, stabbing a guy your own strength or slightly weaker is going to be hard in this pose.
Getting loose from this is fairly easy if you twist through the thumb, but that doesn't improve your position as much at it could. And yes, you can use your knees and feet, but again, for the same of argument, let's say the first thing you want to do is disarm the guy, which would tip the scales in your favor.
What's a simple way to do it?
Easy. Turn slightly to your left and as you do, stab him in his knife arm before he can get it loose.
Assuming you haven't forgotten the lecture earlier on the page, you should be able to see why this will work, but I'll remind you: Face on and stabbing against the body pits your strong muscles against his strong muscles. Changing the angle of the stab from forward to the side pits your strong muscles against his weaker muscles. Chest, arm, shoulder -- against shoulder, rotator cuff, arm.
The trick is, to do it first before he thinks about it ...