How to use it
Pick a preset or type your own numbers, check the total at the bottom of the display, then press Start. The timer runs warm-up, then every work and rest pair in order, then the cooldown. Sound carries further than a screen does, so the phase you are in has its own tone: a rising two-note cue into work, a lower single note into rest, and three beeps when the whole session ends. The last three seconds of every phase tick, which is your cue to get into position.
The countdown is anchored to the system clock rather than to a ticking counter. It reads the real time on every frame and works out which phase you should be in, so locking the phone or switching apps does not make the workout drift. Pausing stores how far you had got and rebuilds the schedule from the moment you resume. While a workout is running the page asks the browser to keep the screen awake, which most mobile browsers honour if the tab stays in the foreground.
The presets are the common formats rather than prescriptions. Tabata is 20 seconds hard and 10 seconds easy for 8 rounds, from Izumi Tabata's 1996 study on speed skaters, and the protocol only produced those results because the work intervals were run at roughly 170 percent of VO2max. Anything gentler is a 20/10 interval workout, which is fine, just not the same thing. EMOM means every minute on the minute: set rest to 0 so each round starts one minute after the last, and whatever time your reps leave over is your rest.
Interval work is demanding on the heart and joints. If you are new to it, coming back from injury, or have a heart condition, start with longer rests and fewer rounds, and get medical advice before doing maximal intervals. This page times the work, it does not judge whether the work suits you.