It stores a lot of energy and can maim or kill. Release the energy slowly, and stand on the opposite side, if you don't use a spring compressor.

It's important to remember, though, that the best laid plans of mice and men gang oft agly. 9 times out of 10 you don't need jackstands either, but applying Murphy's principle, you can be sure that just when you're overconfident that the car gods are always with you, the car will fall off the jack or the jack will fail.