[Jack21]

First of all, you're using decent, well constructed jackstands.

Engine torque with the engine running has no effect on the car on jackstands unless someone is revving the engine. Even then, the jackstands will hold the car in place. If it were me, I would lift the rear wheels off the floor to insure that no forward or rearward motion could be imparted to the car with me under it.

The worst hazard to you under the car while the engine is running is if you have the flywheel or flexplate exposed. The flywheel teeth will do a nasty job on hands, forearms, and forehead.