Hard training breaks down muscle tissue,proper nutrition and rest build it back up and repair it.
I think a person should train heavy for the amount of reps they are shooting for in a set, whether it be 3 reps or 20 reps,doesn`t matter.........it should be a challenge to eke out the last few reps or more,depending on the number of reps per set.
Try using Giant-Sets for an example..............4 exercises in a row with no rest between movements...........hard to do, and you won`t be able to use maximum poundages, but you will break down muscle tissue if you are pushing hard as you should be.
Break down tissue through hard training,not necessarily heavy training, as in trying to gain more power.
Eat,rest,recover,sleep = repair and hopefully growth.