How long a muscle is genuinely under load during a set. The simplest way to make a set harder without adding weight.
Lifting for two seconds and lowering for two accumulates roughly 40 seconds of tension over ten reps — swinging the weight up and dropping it might give you 15. Same load. The lowering phase is especially potent: controlled eccentrics create a lot of stimulus and are the part almost everyone gives away. A practical starting tempo is about 3 seconds down, 1 second pause, 1 second up. Keep it in perspective though: TUT doesn't replace progression, it's a tool for phases when adding weight isn't sensible — around injuries, with limited equipment, or while technique is still settling.