The Rackside Apple Watch app puts your rest timer on your wrist, so you can coach without picking up your phone.

Rackside for Apple Watch installs with the iPhone app. If it’s not on your watch, open the Watch app on your iPhone, find Rackside and tap Install.
The watch shows the countdown ring. Tap +15 s to add time or Skip to end it. Your iPhone stays in sync, and your watch taps your wrist when the time is up.
When nothing is running, the watch shows quick-start buttons:
Rest buttons for 60, 90 and 120 seconds — your default rest is highlighted.