Doing supersets, drop sets, 21's, flex-n-hold sets, partial rep sets, forced rep sets, and negatives are all different shock techniques that stimulate growth by pushing the target muscle beyond failure. Different people use them differently, but I like to throw them in every few workouts on any given target muscle just to keep the intensity up and to keep the muscles from getting adjusted to my workouts. I'd advice using those different techniques sparingly, as you can overtrain much easier if you do these too frequently. When I'm "off", I don't do these all that often just because you push the muscle so hard that recovery can become more lengthy.
SUPERSETS
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A superset is when you complete a set of a given exercise then jump right into another exercise without a rest. It could be two exercises targeting the same muscle group, or different groups. For instance, if you do legs all together on one day you might try doing leg curls followed immediately by leg extensions. Or, on a shoulders day you might do a set of military press followed immediately by lateral dumbell raises. The goal of a superset is to either a) increase overall metabolism and/or b) really push a target muscle to complete failure/exhaustion. When cutting, I use supersets quite a bit because it turns a resistance workout into a cardio workout.
Let me know if you want any more input on the other shocking techniques. Good luck with the training!
