Looks painful, right? You are probably asking yourself why would anybody do that. Well, the idea is that over time the wrist will get very strong and conditioned. Gymnasts, breakdancers and martial artists seem to like them. However I would never recommend this type of wrist sex to anybody who has ever had history of wrist issues. In my opinion the risks outweigh the benefits for most people..
You can get brutally strong wrists doing many other exercises and should I say much safer - knuckle push-ups are a good alternative. However I know some of you are nuts and strong and will try it anyway ;)
The main benefit behind knuckles push-ups is the added wrist stability. The wrist is in much more natural position and there is less strain. At the same time all major ligaments and muscles of the forearm are involved in the exercises. Make sure you do your knuckle push-ups on the first two knuckles just to make the exercise harder. However make sure you use soft surface for your knuckle push-ups, at least in the beginning, in order to avoid damaged skin.
And if you are feeling really crazy you may try this stuff. I personally have better things to do. Have fun kids.
Conclusion: Knukles push-ups are much safer in the long run.