During my young teenage years, me and a couple of friends of mine went biking. We found a place where we could do some cool ramp stunts and whatnot not far from one of said friends' house. We got going to speed and we made some nice air on some of our jumps. However, one of my jumps ended in an almost Darwinian fashion. When I landed, my butt missed the seat by maybe a half an inch...and my nuts found the rear tire. I still had a good speed going when I landed and my hands weren't near the hand brakes, therefore I wasn't able to reflexively hit them. Moreover, my grip was such that, even if I had tried to brake, I would have fallen off the bike and taking the risk of injuring myself even further if I had attempted to move my fingers. So, in short, my nuts became the brakes for what seemed an eternity. When I stopped, my genitals felt as if they were on fire and quite tender (later, after getting home it was discovered that the friction of the tire wore through my shorts and almost through my underwear) and my stomach was in a constant vice-like state and I felt like I had nausea for almost two days. I couldn't walk correctly for a day, eating was severly limited and trying to take a piss was like trying to shoot needles out of my system.
Nowadays, I steer clear of attempting bike stunts. So to speak.