For short, high-intensity intervals such as Vo2max efforts and sprints, we recommend turning off ERG mode on your indoor trainer. This allows you to push to your maximum effort without being limited by the trainer's resistance. The power targets provided by JOIN are a useful reference, but turning off ERG mode gives you the freedom to go all-out.
For longer intervals that require steady, sustained efforts, like those at FTP or Tempo level, it's best to keep ERG mode on to help maintain a consistent power output.