Set a rep cadence, i.e., at least 5 seconds to lower. When you can no longer lower in 5 seconds, terminate the set. Do not continue until the weight drops. Doing so will inroad your muscles too much and increase the risk of overtraining (happened to me in only two weeks of 2-3 sets per muscle negative style).

Make certain the lowering is even... don't spend extra time in the "easy" zone, then allow the weight to pick up speed for the remaining few seconds. If you can't lower it under control in the hardest zone, then terminate the set (or reduce the weight).

Avoid multi-joint exercises for negative only training, unless using a machine... it's too dangerous to have others lift a heavy barbell for you, i.e., bench presses or squats.

Brian D. Johnston