It's just a timer, which I set for 3 to 5 minutes. When the timer reaches 0, (part of) a particular song plays. (And a record is made of the start and end times of the meditation session.)

In the rare case that I am doing something other than meditating when that song plays, I know I got so distracted that I did not even notice I got distracted -- which I take as a sign that I have an unusually pressing need for meditation that day.