It’s the simplest thing to do on the site, and it’s the most amazing. To begin slice playback, click the play button in the lower left of the control bar. Pausing stops the playhead.
If your slice has a video recording, you can click on the video itself to play/pause.
Finally, if you’re on a computer with a keyboard, you can use the Spacebar to play/pause — as is standard in music/audio programs.