A clock between sets, set per exercise. It starts itself when you log a set and counts down where you can see it without leaving the set you are about to edit.
Logging a workout is free, and so is this.
Setting it
On any exercise in the logger, open the menu and pick Rest between sets:
No timer, 30s, 45s, 1:00, 1:30, 2:00, 3:00, 4:00.
It is per exercise, not per session. Heavy squats and cable curls want different clocks, and the setting sticks with the exercise inside that session.
The two places it appears
The bar sits at the bottom of the logger. It is what you see almost all the time: the number, out of the way, while you edit the next set.
The sheet opens when you tap it. Big numerals, and the controls to change the length or stop it.
It survives your pocket
The countdown is worked out from the time it will end, not by ticking while the screen is on.
So locking your phone, taking a call or switching apps for four minutes and coming back shows the truth rather than a clock that quietly stopped. Rest three minutes with the phone in your pocket and the timer knows.
Turning it off
Pick No timer on that exercise. Nothing starts and nothing is shown.