vehicular convulsions

On my way to church this morning, I stopped at an intersection and waited for the light to change. It was a long light, and I noticed an ambulance also waiting at the stoplight (without its lights on). Suddenly the driver of the ambulance started "rocking" the vehicle by easing off the brake and then stomping on it again and again, so that the whole ambulance began jerking back and forth. It looked like the vehicle had suddenly gone into convulsions. It continued convulsing for several minutes until the light turned green and we both went through the intersection. It looked like those kinds of things that you see teenagers do with their buddies....and something that I never thought I'd see an ambulance doing. Who knows...maybe they were in desperate need of ambulance drivers this morning and had to fall back on their teenage volunteers.