What the negative is

Every rep has a positive (lifting) and a negative (lowering) phase. Controlling the negative keeps tension on the muscle and is part of honest cadence on any single set to failure.

Accentuated negatives

Once you reach momentary muscular failure on the positive, a partner or the working leg can help you back to the top so you can keep lowering under control. It extends the set past positive failure, in the same family as rest-pause.

Where it fits

This is an advanced intensity technique, not a default. Mike Mentzer folded eccentric work into Heavy Duty for exactly that reason: a little, applied late, when a straight set no longer challenges you.