Article by Emanuel Giordano

The conditioning exercises, in Okinawan Karate, are essential to train the body and prepare it to receive various shocks, causing as little damage as possible. If performed correctly, it is a practice that does not lead to damage or impediments to the human body, which is why it should always be learned by those who have learned it directly from their masters, avoiding practices tending to fanaticism, whose sole purpose is that to grab attention.

Obviously these exercises (video) concern many parts of the body, but in particular those aimed at offending and / or defending. Clearly the more you have trained (correctly), the more your "weapons" will be strong, also giving you greater self-confidence. In particular, one of the parts most exposed to impacts is the wrist and, in this case, the hard part below the joint. At this point, the radius and ulna bones are more exposed, which is why proper training / conditioning (kitae 鍛 え or tanren 鍛練) is recommended.
Miyagi Hisateru, in his book Karate-do (1953), reports one of the exercises practiced by Yabu sensei. Below there is an informal translation and the original text:
Most blocks [uke] are done with the wrist. This is why it is important to train your wrists. Mr. Yabu (already presented in another section) was taught to alternately beat his wrists up, down, up and down. This is a wrist training method, so even if you defend yourself against another person's punch, your hand won't get hurt. "
Original text:
受けの大部分は手首による。だから手首を鍛練するこマJ は大事なこごで ある。屋部先生(別項に紺介)は自分の両の手首を、上下上下ど交互にたたき合うように教えられた。これは手首の鍛線になり、他人の拳を受けても自分の手が痛くない。"
  • "Shorin-ryu Karate: kata 2" (here)
  • "Shorin-ryu Karate: kata" (here)
  • "Shorin-ryu Karate: The legacy of the bodyguards of the king of Okinawa" (here)
  • "The legend of the masters of Okinawan Karate: Biographies, curiosities and mysteries"  (here)