A repetition of an exercise has two parts: The concentric (positive) contraction shortens a muscle, and the eccentric (negative) contraction lengthens it against a load. In a biceps curl, for example, curling a weight up to your shoulder is the positive portion and lowering back to the start is the negative.
Your body is much stronger in the eccentric than it is in the concentric contraction, and I take advantage of this by integrating negatives into your finishers. You’ll see them programmed as taking two seconds, so from the top of the move all the way to the bottom should take you the entire two seconds. Make sure you stick to that range for optimal results.