If you have the "Track TUT" option enabled in the app settings, you can not only mark sets as completed but also start them and record the TUT for each set.
The main usage scenario is assumed to be automatic. The metronome will open automatically when the set starts if the tempo is specified. If the repetitions parameter for the set is known, then after you reach this target number of repetitions, the set will be automatically finished. If the rest time between sets is specified, then the rest timer will automatically start, and when this rest is over, the next set will start automatically (the metronome will open) - and so on.
Also, two new settings appeared: Countdown and Delay.
If you start a set using the button (manually or by skipping the rest timer), a countdown will launch to give you time to prepare for the exercise (take dumbbells, take the starting position, etc.), and start the set after the signal. If a set launches automatically after the rest timer ends, there will be no countdown since you are assumed to start the set immediately after the timer beeps.
If you finish a set manually, some time will pass after you stop doing the exercise and before you press the Done button. That is the delay - and it is taken into account when calculating the time under load - that is, when calculating and recording this value for each set (you can also record this value manually).
You can set general default values for these Countdown and Delay settings - for example, 5 seconds. Also, for each individual exercise in a workout or routine, you can specify more precise values specific to this exercise and the conditions in the gym.
There is no particular benefit from tracking TUT yet - but in the next release, there will be a widget for monitoring the total time under tension with a breakdown by muscle groups where this data will be used (as well as a similar widget for the total number of working sets).
Tip: if you are using an Apple Watch and find that sound (and tactile feedback) is not loud (strong) enough - please try to increase the volume in Apple Watch settings.