ERG mode is a feature on smart cycling trainers that allows either the app you are using, or the trainer itself to set the resistance you experience. For example: You are doing a tempo session. ERG Mode will “lock in” the trainer at your tempo range so you can stay at a constant tempo power output. It gathers this data from FTP number you have set in your app(s). It is compatible/available in most smart trainers or apps (i.e. Wahoo app, Zwift, Garmin, Training Peaks Virtual, etc.)