I would never go under a jacked up car. Just use the jack to change the tyres that is it. Anything else I take it to the mechanic who will lift it with the proper machinery.
Oh indeed, most people wouldn't. I bet some people do though.
Especially if the car is broken down in the road (which is where cars usually break down), and there is no tow truck but someone has a jack (e.g. the basic jack for changing the tyre) and there is pressure to get the car off the road. It's inevitable that out of all the millions of cars and people, someone will be in
a hurry to jack it up using the spare tyre jack and not much else, climb under, put it in neutral without thinking of what will happen next, and get crushed.
