Complex sentence: Treadmills often use a safety key (magnetic based) that the runner clips to his or her waist, so that the safety key is pulled out if the runner falls, and the machine stops immediately.
Split into simpler sentences: Treadmills often use a safety key (magnetic based) that the runner clips to his or her waist. The safety key is pulled out if the runner falls down, and the treadmill stops immediately.