An additional 'open' step, i.e. a step without a predefined end, added onto the end of your workouts. This allows activity recording to continue until you manually end the workout, giving you extra time to run back (hence the name) home.
Perceived Effort Assessment's have the run-back added before the final standing 2 minutes.
Run back steps can be enabled / disabled from your web Profile page (for watches and external services), or the Workout Settings screen within the TrainAsONE mobile app.