How a lifter's force capacity changes through the range of an exercise, which is why every lift has a hardest position.
Three shapes cover most exercises. Ascending: strongest at the top, like a squat or bench press. Descending: strongest at the start, like an upright row. Bell-shaped: strongest in the middle, like a biceps curl, which is easy at the top and bottom and hardest at 90 degrees.
Free weights load the curve unevenly, so the muscle only meets a hard challenge at one point. Cables, bands and some machines flatten it. That is the honest argument for machine work in a program: a cable curl keeps tension on the biceps at the top of the rep where a barbell curl has none.
Practical use: pick exercises whose hardest position matches where you want the client to get stronger. If a lifter is weak at the bottom of the squat, a pause squat trains that position specifically.