Both inside and outside of Aikido there has been some confusion as to the purpose of wrist grabbing practice (katatedori).
It’s tempting to explain wrist grabbing as a form of self-defense against someone trying to grab you and the technique as a method of trying to get back your grabbed wrist. Given the fact that the practice methods of Aikido arose from the aiki-ju jitsu tradition of the Japanese samurai it’s unlikely that a battle hardened warrior would concern themselves with training against such an inconsequential attack. This simplistic view of wrist grabbing as a literal form self-defense training is misguided and misses the history, the spirit as well as the real practical value of this practice.
Arm grabbing techniques were an actual part of feudal swordsmanship in Japan. If an opponent approached you prior to your drawing the sword then your draw might be temporarily prevented if he could effectively stop you from moving your hands to draw. Swordsmen practiced movements to both foil another attacker from drawing as well as to defend against someone who tried to stop them from drawing.
Aikido is a sword art, even when the sword is not being held. A true aikido practitioner will always embody the concept of the sword in their movement. So these arm grabbing techniques are the bridge between actual sword-to-sword and empty handed techniques. Unfortunately, even some instructors in the aikido do not correctly embody the sword in their movement so the wrist grabbing practicing becomes too focused on the wrist itself.
There are other benefits to this kind of training as well. The technical principles of ju jitsu, judo and Aikido rely on the skilled use of movement, balance, leverage, alignment, timing and spacing. These arts derive their effectiveness from coordinating the attackers force and motion into the technique and breaking their balance. The process of learning to implement these subtle skills requires grabbing or grappling to really get the sense of it. The ability to feel the dynamic shifts of balance and movement of another person requires developing awareness on many levels not generally encountered in other areas of life or athletics. Mastering your own dynamic balance while simultaneously leading and controlling the movement and balance of another person becomes a highly specialized skill in Aikido.
~Philip Greenwood, Sensei