A good body shot doesn't need to be hard to put someone down. If you rewatch the shot that Khan hit him with it was a left hook to the body. That's a liver shot, probably the most painful thing that can possibly happen to somebody in a boxing ring. Imagine getting hit in the *****, except the pain is literally about 10 times worse and instead of just your ***** and stomach hurting it's every inch of your body. It feels like you're going to die. It's also pretty much impossible to move your legs, the fact that Maidana was able to get up after that and survive is a testament to how tough that guy is. Look at B-Hop vs Oscar, that shot had nothing on it yet Oscar was rolled up into a ball screaming, it doesn't take much as long as the punch is well placed.