How to use the online timer
Seconds are the right unit whenever a minute would be too blunt. Interval work lives here: twenty seconds on and ten off, forty and twenty, the fifteen-second transition between circuit stations, ninety seconds of rest between heavy sets. So do the small kitchen moments, a soft-boiled egg that wants an extra twenty seconds, and the two minutes the NHS asks for at the sink.
At this scale, the tightness is usually the point. A ten-second rest is meant to be barely enough, and stretching it to fifteen changes what the exercise is doing. Counting in your head drifts within a round or two, particularly when you are out of breath, so a countdown that holds the number exactly is the difference between the format you intended and an easier version of it.