Why would someone want to expose themself to serious injury or death just for a few thrill-inducing minutes jumping off a bridge and bouncing around uncontrollably? Yes, we've all heard the catch-all justification: "It's exhilarating -- and it makes me feel alive." Here's a closer look at the health consequences of this edge-of the-envelope activity. On its face it seems quite risky, but how risky is it? Maybe not as much as you think.