One particularly evocative way to imagine Aikido is as an exchange of various forces. For this exchange to be realistic the correct execution of Ukemi is vital. During practice the Uke attacks and the Tori responds with Aikido. The Uke's attack and response to the techniques performed on him is described as Ukemi. Superficially this is a matter of rolls, break-falls and sincere attacks. The Uke's role throughout a technique is far from passive. There is the matter of reacting realistically in the midst of an attack to whatever is happening. In this way Ukemi also heightens awareness and reflexes.
There are primarily two methods of practice that require two distinct kinds of Ukemi. Kihon and Kokyu. Kihon means basic or fundamental. Kokyu means breath or breathing. As a general rule when we practice Kihon the Uke grips firmly and does their utmost not to move. When we practice Kokyu a more dynamic attack is defended against. Any grip should be as inexorable as the grip of a vice but it should also be light, it should never crush the Tori. This not only makes practice more pleasant, but also adheres to the fundamental principles of how an Aikidoka should use their muscles.
An Uke should abandon a grip only when they feel that they would, as an aggressor, be compelled to do so. Whether that is to protect themselves by rolling, to defend against an Atemi or to continue the attack. Appreciating when that point arrives comes with practice. If as an Uke you are unsure, particular in the beginning, then go with the flow. It is important to remember that a Tori rarely makes much progress in their Aikido if the Uke is constantly pointing out their mistakes. This responsibility should be left to senior students and the Sensei.
There are people who seem to believe that a firm static grip is the only realistic way to attack. This is to misunderstand practice. Kokyu techniques are often practiced with an aggressor trying to move the Tori in some way. Which gives them some force to work with. As explained in the text on harmony, the Uke also comes with some expectations. A punch is expected to make some sort of contact, a push or a pull is expected to deal with some resistance. Aikido techniques frustrate these expectations and the aggressor is unbalanced. The Uke has to mimic this behaviour to be 'realistic'. This does not mean that the Uke should give himself over completely to the Tori. It takes time to develop the correct balance, not so soft as to be meaningless, not so hard as to be meaningless.
Once someone has lost their balance their first instinct is to regain it. With the lead the Tori attempts to use this instinct to keep an aggressor off balance until they are neutralized. If the lead is correct an aggressor will often feel that the only solid thing they have is their hold on the Tori and consequently they hold tighter, thinking this is the best means to regain their balance.
The lead is an offer. The Tori offers the Uke an opening, something to attack. The Uke rushes in only to find that the opening is no longer there. If the lead is continued in the correct way the Uke continues to chase the opening which they have focused on, or perhaps a further offered opening. The Tori uses this to lead the Uke into a vulnerable position. When an Atemi enters an aggressor's space the aggressor reacts to it or gets a knock to react to instead. That response either has the aggressor putting themselves into a position where it is very straightforward to take their centre, or has the aggressor reacting with force, which gives the Tori something to work with, something to redirect.
If the attack is sincere and the lead is performed correctly, a Tori will rarely feel the Uke become heavy or immovable. Should this event occur it is a simple matter to distract the aggressor with an Atemi and begin again to lead.
The Uke's task is to attack in a manner that is considered realistic. As mentioned in the notes on practice, the essential primary stage in learning Aikido is choreographed. Techniques are practiced according to a strict understanding between Uke and Tori. The Uke is asked to attack in a particular way, the Tori is asked to defend in a particular way. This is a very efficient way to wire the building blocks of Aikido into your body whilst keeping the chance of injury to a minimum.
The problem for the Uke is that they are aware of what the Tori plans to do and they must behave as if they were not. They must try not to anticipate the Tori's defence during this kind of practice. An Uke's attack should always be fully committed, sincere. An Uke should focus on their target and try to hit their target. Again in normal practice you might find yourself anticipating the direction in which the Tori is going to move. This makes for meaningless practice. If your attack on the mat is insincere, is going off target, you cheat your partner. If these habits were to continue the Tori's Aikido would be seriously flawed. An aggressor outside the dojo has no idea what is going to happen, where their victim is going to move. They only know what they want to hit. On the mat we must simulate that.
There are various stages of Ukemi. In the beginning it is important to give yourself over to being led. Again, being careful not to anticipate. An Uke should give something to the tori and then move where they are directed. Later Ukemi is different. Once a Tori's body has absorbed how to move, we progress to the Tori responding with a variety of techniques to one attack, so the Uke no longer has to worry about anticipation. Then to a variety of attacks with the same finish, then to a variety of attacks with a variety of finishes, all the while developing a sensitivity through practice to the movement of the Uke and beginning to feel what is appropriate to the situation, what the Uke is offering to you.
The more you practice the more responsibility you assume as Uke. Your attack does not end with the first cut or initial hold. You look for weaknesses or openings in the Tori's defence. After Dan gradings there is also Kaeshi Waza, where as Uke you are constantly on the look out for an opportunity to reverse the situation and become the Tori. But that is much later. Practice Kaeshi Waza too early and you are likely to slow your overall development. It takes time to understand what is and what is not a legitimate opening, to appreciate what is truly realistic. But this is actually a simpler matter than it may sound here and naturally emerges from regular practice.