Negative Rep Training
The principle is to carry out only one phase of the movement, that which is called negative or eccentric, ie the descent.
Since the muscles are indeed stronger during this phase, it is possible to use heavier weights (10 to 20% more) and therefore gain more effectively in strength.
The negative phase must be performed without assistance, very slowly, making sure to constantly brake the bar.
For all exercises with a bar this technique requires the help of a partner to carry out the ascent phase. In the movements with a dumbbell you can help yourself with the hand that remains free and give an impulse to reach a high position. Ditto for certain movements on the machine, including with the legs.
A low number of negative repetitions is very often enough to stress the muscle. As with other intensification techniques, these repetitions tire the muscles a lot and must therefore remain occasional.