The extension app can start, pause, resume, and stop your workout. Moreover the user can view current status of the workout like distance, duration, pace or speed, and heart rate. This is very handy when the phone is in your arm case or bag.

This workout page (1/3) shows the distance, duration, and sport type. User can swipe left or right between the three workout pages. Notice the pause watermark / icon that the user can press to pause the workout. 

Workout page no 3: heart rate. 

The summary shows the most relevant fields.