BDSM
How much abuse can the nuts take? There are funny youtube clips about a fake reality show called "Kicked in the Nuts" Black jumpsuit guy in orange afro.
Now I see tons of clips of kids intentionally falling on a fence or railing.
And if you really go medieval, "Ball Busting" is like watching a car crash. You are horrified but can't look away.
It seems to be the ultimate in helplessness. Being completely restrained with the family jewels just dangling there. The expression, "got him by the balls", comes to mind.
I have looked at some medical info on testicle trauma, but it does not take into account intentional and repeated abuse.
I have the added complexity of having a couple of metal clips in my sack.
Is this dangerous?
OH, right, James Bond ended up in a hospital
Yes, James Bond did wind up in a hospital, and you might recall who put him there!
The secret to this type of play is that the kicker is NOT following through with the kick. The kicker is pulling the kick a little (just like stunt people pull their punches), and also striking not with the toes, but generally with the flat part of the foot above the toes. It's generally to the benefit of the kickee to keep/hold their manhood out of the path of the kicking. You can either do this type of play with the kicker facing the kickee, or a better way might be with the kickee on all fours facing away, and you kick between the legs, under the butt cheeks.
It's just another form of impact play and, unless you're doing it way to hard, it actually kind of a nice mixture of discomfort and pleasure. Not significantly worse that a flogging to your junk. I've have it done to me, and have yet to piss blood.
There are some BDSM videos where the kicker is kicking full force, but I wouldn't consider that representative of this type of play. As with all things kinky, pre-negotiate how hard you're willing to be kicked.
I have to admit that I am a CBT addict...I am constantly amazed at how much a guy's balls and cock can take. Fortunately there are relatively safe ways to do a lot of the things I like to do, and, since it is a pretty sensitive area, it doesn't take a whole lot of work to get the desired result.
I don't really care to "kick" as much as I like to "squash"...not really trampling, but more of a persistent pressure that gradually increases until the bottom taps out or I can see that it is time to stop (purple is not a good color for any long period of time). I have excellent tools designed for this very thing, as well as my hands, and my feet 
And, since you are not doing damage to either the bladder or the kidneys, I don't think pissing blood will happen...now, rupture of the testicles could happen from too much pressure, but I like having them around to play with another day