All you do is press L3 (press in, no direction involved) right before the CPU snaps the ball. Make sure you have selected the defender you want to lock on with. I suggest a DE or a LB to start. Single most important reason I prefer NCAA over Madden.
Also if you're in a dynasty and you recruit your first 5* or 4*, it really helps you take advantage of their talent if you lock on to them.
But yes, it is ****ing awesome. I use it every single play on defense. Using it as a DE is awesome, because sacks and forced fumbles are so rewarding. L1 to tip passes at the line. Watch out for those screens!
What else does what? I am going to try and play D End.