My dad had a motor mount break on a '65 Impala SS. One minute we were sitting at a stop light, the light went green and next thing I know the old man was burning rubber across the intersection Dad reached up and shut the car off and we coasted over to the curb. It was a manual trans. car and when the motor mount broke, the engine torqued over, throttle went wide open and the clutch and foot feed went to the floor. Only option was to turn it off. I gotta say I was impressed at how quick the old man shut it down. He wasn't trying to do a burnout, he merely just let out the clutch and things went "up in smoke" real quick! He took it to the local Chevy dealer and they replaced the mount. The new one had a safety cable around it, so if it did break, the separation was limited.