During an offline workout, the double tap will add a placemarker in the workout history. For example you can double tap when you start the workout, end the workout, or to show breaks or lap splits.
NOTE: This feature is only available in the iOS Wahoo Fitness app.
During a workout with your phone the double tap will control your music playback by default. However you can edit this to perform other functions under the workout settings (Settings > Workout Profile of your choice > Double Tap).