What the heck is the mind-muscle connection? It's as simple as concentrating on the muscle you're using when you do a particular exercise. It's especially important in back and ab exercises, because it's easy to crank these out by the dozens if you're using momentum and not activating the correct muscles.
The way to do this is to think about initiating the movement with the muscle group you're working. Let's use seated rows for example. Pretend your hands have no strength, they're just hooks. Instead of pulling with your arms, envision squeezing your shoulder blades together to make the weight move.
For abs, a good example is leg raises, either lying or hanging. Your legs have nothing to do with the exercise. Tighten the abs and use the contraction to get your legs up...they're just along for the ride.
Using this method makes the exercise harder but you will FEEL it so much more and get much better results!!