Ukemi- the ability to land on the ground without getting hurt, the ability to "receive" an attack from a training partner, the ability to blend and flow a movement. As a foundation in the ten ryaku no maki section of training it is a dual skill- one we practice in every class, and one that is on the roster for daily training at home.
The focus this past class on ukemi was building an awareness between ourselves and our training partner, and when is the moment to take ukemi- from the perspective of preventing injury. As an example, we used omote gyaku to explore the idea since it appears in many places in our training- the kihon happo, gyaku gi, and a number of kata in the jin ryaku section of training.
In this movement, your training partner takes your wrist, and using correct taijutsu movement, locks up your body and ability to move. Through the twisting of the wrist and the arm, there is a slowing of ones ability to move as it locks the elbow, shoulder, hip, knee, and ankle of the lead foot. There is a moment, where the body entirely locks up and a moment later you are thrown over with the omote gyaku movement, done is such a way that one can't take ukemi.
When we are practicing ukemi, in that precise moment before the entire body is locked up, we use on of the break falls or rolls to land- depending on the technique and the position of the training partner. The idea is to be sensitive enough to the movement to know when to take ukemi or not.
Done early your training partner will just change waza and take you in a different direction, done late, and you can't escape or take ukemi, done at the right moment you blend and can protect yourself from the technique.
From there you can also perform kaeshi waza or sutemi waza to capture your training partner with their own technique.