Time under tension (TUT) refers to the amount of time a muscle is held under tension or strain during an exercise set. During TUT workouts, you lengthen each phase of the movement to make your sets longer. Healthline 
My First Million - "You shouldn't be trying to get those done, all the gains are in that last bit. That's where you should show poise and go even slower than your normal pace. When you feel that sizzle, the reaction shouldn't be panic and rush, it should be poise." Listen