The safety key has a magnetic key side and a clip side. The key side should remain attached to the Peloton Tread or Tread+, while the clip side should be attached to your shirt or pants while using the Tread. If you need to stop the Tread, pull the string to pull out the key. Once you do this, the Peloton will come to a stop.
Another way to stop the tread is by clicking the stop button located on the handrail.