Simply put, use the muscle not momentum. Vince or Nick would be quick to point out when I was cheating myself by using momentum instead of focusing on using the muscle to perform the movement. This also means don't "explode" at any part of a movement.
Usually I was aiming at completing a perfect 8 reps, but sometimes they would yell at me to keep going by doing "burns" - which consist of performing quick, short range of motion reps (focusing on the contraction part of the rep). Otherwise I performed reps in a very strict and focused manner.
Quick note: I remember Nick talking about one of the real old school guys (sorry I can't remember his name - but I think he called him "pumper" or something like that, cause he only tried to get a pump) who would show up at Vince's and use light weight, fast reps, and no real form. According to Nick the guy was huge. His point was...different strokes for different folks.