When I 16, I was under my 56 Chevy (can't remember why), had only a floor jack holding it up. Didn't realize it was slowly leaking down. Before I knew it, there was enough force on my chest I couldn't scoot out from under it! Some wild screams brought my mom out into the garage and she gave the jack a couple pumps so I could get out. If she hadn't heard me, I probably would have suffocated. I always support with jackstands if any part of me is even close to being under the car, I leave the jack positioned too!