Warren,
Over on the FCA site, they have a video of some guy that his wife took and he has his car propped up similar to that. They said about 2 minutes after he crawled out from under it, the bumper jack slipped and it came crashing down. I had to edit this as I had said he used a 2 X 4. I am attaching a picture of the way he was just before he got out from under the car and it fell.