A nice article by Jeff Sodeman from the Aikido Pinner website.

The Three Purposes of Ukemi

A good place to start in thinking about and discussing the topic of ukemi is an exploration of the purpose of ukemi. In general ukemi has three roles: creating a situation for the nage to practice, protecting yourself as you take falls, and finally learning about aikido through taking ukemi. They say that “ukemi is 50% of Aikido” and that doesn’t just mean the time you spend in that role, but also the depth and principles involved.
Learning Aikido through Ukemi
On the surface, learning through taking ukemi would seem to mean watching and feeling “How is my partner throwing me?” This is valid, and there are many subtle things in Aikido that have to be felt to be understood. There is also another opportunity for learning the principles and movement of throws by receiving them.
When you attack the nage you have your first aiki interaction, in this one the nage explores aiki through performing technique. The second interaction is when the uke receives the power of the throw from the nage. In exploring how to receive that power you can learn a lot about aiki principles. Finally, you interact with the ground. If you look for ways to transform your energy as it comes into contact with that mat, rather than simple slamming in it, then you have a final opportunity to explore aiki principles. So really when you think about it the uke has twice the opportunity to practice Aikido as the nage in every single throw.
Protecting Yourself
This is probably what most people thing of when they hear “ukemi.” The physical technique of taking a fall. Most people are taught 4 basic falls: back fall, front roll, ikkyo ukemi, and breakfalls. With experience you begin to modify them and adjust them for different situations, but I think these variations on falls aren’t often practiced in a methodical way, and some of the technique posts here will help to address that gap. Each throw has a fall that perfectly matches it. The more flexibility you have in falling, the closer you will be able to get to receiving throws without trying to change the fall into something you are more comfortable with.
Helping Nage
Our main purpose in being the uke is to create the proper situation that allows the nage to work on whatever it is that’s being practiced at that time. Really, if we’re not there for them then we have no reason to be on the mat working with a partner as their uke. Creating the right situation is not a trivial thing however, and in fact it can take years of experience and a solid understanding of the throwing side.
Ellis Amdur and others have pointed out that in most traditional martial arts, it is the senior person who takes on the role of uke both because they’re better equipped to handle the unexpected from a beginner, and also because they understand what is being practiced. For whatever reason Aikido generally does the opposite, with the beginner put into the uke role first.
Ukemi has two parts, the giving of the attack and receiving what the nage does with it. In normal practice we want to interfere with what the nage is doing as little as possible, attempting to give our body and energy to the nage for their use in exploring a technique. In reality our ability to do this well is limited by two things. First, our technical or physical ability to receive the fall. If you are thrown in a difficult direction for example, do you have to take control back from the nage to turn the fall into something you can take, or are you able to take whatever they give you. Second, can you emotionally or psychologically give your trust to the nage and give up control to them.
Giving up control to the nage is one of the hardest things to develop. If at some point in the technique the uke decides when to take the fall, what kind of fall to take, what direction to fall then they’ve take control back and are no longer receiving the throw, but (while it may be very subtle) are now throwing themselves.
Additionally, many of us fall into the trap of thinking that our job as uke is to challenge the nage. It’s meant in a constructive way, and there are certain specific types of practice that it is appropriate for, but challenging the nage generally hinders their ability to learn. Each technique fits a specific situation, and if the uke changes the situation in trying to provide a challenge but the nage still has to do the original technique then you the technique no longer works correctly. This can lead to tension, over use of muscle, and lack of confidence as the nage.
So, if you don’t challenge the nage then how do they grow? When practicing specific techniques, as you’re able to give yourself up to the nage more and commit more to your attacks, the nage will have more of your weight and energy to deal with. Stability, structure, timing, movement, and an understanding of the technique grow out of this. The other way to constructively challenge nage is through free-attack free-technique practice, in which the nage gets to learn to pick the correct technique to apply to each different attack.
I think we convince ourselves that Aikido technique is difficult to do, and a lot of this comes out of struggling with an uke who isn’t giving us what we need to practice correctly. If you’re doing the right throw for the right attack, then technique becomes very easy to do.
This is a large and complex topic, one that I’ll explore more in further posts. For now I’ll finish with a personal anecdote. There was a large group of people testing on the night that I tested for nidan. At the end of the tests sensei gave some general feedback to the group, but didn’t mention anything specific to me. I almost never asked sensei questions, but this time I worked up my courage and approached him the next day to see if he had any specific criticism. After I asked him, he thought for just a moment and responded “Your ukemi is too fast.” I thanked him and wandered off confused. I’d often been told that I had really good ukemi, and it was something that I worked on a lot and loved doing. The last thing I thought I’d hear was an ukemi comment as my only feedback. Eventually I came to understand that I was taking falls for myself and not for the nage, and now many years later I see the depth and importance of sensei’s comment.
Thanks for taking the time to read this blog, and I hope it makes you want to find out more about Aikido