Forced negative movements are for individuals who are looking for muscle hypertrophy. You are taking an exercise and increasing the weight by a certain amount that you will need assistance lifting it up. When your spotter has helped you lift the weight up, it is your job now to bring the weight down, but as slow as possible. That is why it is called a forced negative because the concentric or positive phase is assisted and the eccentric or negative is all you. The muscle is getting more fatigued with heavier weight, which is what you want when your goal is muscle hypertrophy (gain in muscle mass).