Time under tension - eg, if somebody bangs out lets say with 15kg dumbells some arm curls.
Does 12 reps - 1sec up & 1sec down. = 24sec set.
Then same person does 4 sec up, 4 sec down for 3 reps at same weight - they “should” fatigue at the same 24sec point - but, only did 3 reps!!
It’s so much the reps they did, but the 24sec of exercise.
Try it, just don’t go Pausing between reps - keep moving.