You want to vary the intensity of your workout? This timer allows you to set up a series of higher intensity bursts, along with lower intensity periods in between.
Just choose a number of intervals, interval duration and total time and start the timer. It will track and display what zone you are in and when to change.
A simple series of ticks alerts you to a change in zone (so you should be able to play music alongside it).
On iOS devices with 3D touch there are shortcuts to quickly start standard workouts.