The SYSTM app has an Auto-Pause function that can be controlled from the workout settings panel. Click on the gear icon in the top right corner of the workout player and select SETTINGS to access this screen.
With Auto-Pause toggled to the ON position, the workout will start and stop automatically when you start pedalling. There are some workouts, such as Full Frontal and Half Monty, where you will be required to turn this function OFF. You will need to start and stop the workout using the Play/Pause button in the center of the screen.