To activate the Warm-up and Cool-down segment types for Timefully on the Apple Watch, go to the timer screen on your iOS device (iPhone or iPad). Tap the Settings icon in the top right corner. From there, you can turn Warm-up and Cool-down on and off with the switch below the slider.
Note: The Cool-down segment will not activate if the "Open session" setting is on since open sessions let the Main segment continue until it stops.