This Gen 06 stop button fits some models of treadmills by Precor. 

The stop button is an emergency feature of the treadmill. The button will stop the machine when it is pressed or engaged. 

The safety clip lanyard feeds into the stop button. The stop button fits over the micro plunger switch in the switch mount. When the stop button is pressed, it engages the micro plunger switch. 

You can press the button with your hand while exercising. Another way to engage the button is to pull on the safety clip lanyard.