In a crane cab, the operator had two switches (the round black buttons in the photo). One of the switches activated the microphone so the operator could talk to the rig handlers on deck. The other foot control activated the emergency shut down control on the crane engine. To reset the emergency shut-down feature the operator had to leave the cab and go into the machinery space behind the cab.
Thanks to Gerry Miller for this photo. You can see Gerry's profile on LinkedIn.