Ukemi can be understood to literally mean; receiving body , or receive with the body. It is what uke (受) ; whoever receives the technique; does when nage (投げ - thrower) or tori (取り- grabber) applies a technique.
First, you will learn how to fall from a sitting position, then from a squatting position, then progress from a standing position. There are three core principles to be learned;
Once you have an understanding of the basic principles, with a bit of practice, you will then progress by being thrown gently, by which, you learn in a very unique way, the mechanics of the throw being used and how best to accommodate your body to it in your ukemi. At the start, expect your body to rebel against the unfamiliar feeling of disturbed balance and movement in an upside down position toward the mat.
Flexibility is the ability to stretch, twist, and bend the body. The more the body flexes with the force of a throw, the more you will feel a sense of controlled connection to the power of the throw and the more possibilities you gain to avert injury. Safe and enjoyable ukemi also necessitates conditioning which comes only with frequent practice so that your body learns how to accommodate and best absorb and control contact with the mat. Ukemi is all about receiving technique gracefully, under your own terms so that you can rise up to continue training with ease.
Trust: There is a trust between Tori and Uke. When you are taking ukemi, you accept that you will be placed in a vulnerable position, so you need to trust your partner to apply technique in a controlled manner with only necessary force and speed to allow you both to learn and practice safely. Always talk to your tori and create a good training dialog.
Form. There are prescribed methods for performing ukemi safely for different throwing techniques. Learn the basic forms and tweak to suit your tori's application, your body type, and size.
Relaxation. No matter what type of ukemi you attempt to perform, relax your body as you receive throws and meet the mat. Never hold your breath. Breathe out as you reach the ‘point of no return’ as this will naturally help you to relax. With time and practice your body will learn to relax as you get more familiar with the feeling of the technique being applied; finally with it becoming a naturally stored response.
Blend and Cooperate. Learn to blend with and not resist tori’s technique. Your partner will appreciate your co-operation because it helps him/her to work on better technique and you will find that Tori will need to use less force, making ukemi easier to learn and perform. Resistance while learning technique in kata, kekarigeiko or Ninin-dori does not help you or your partner. Physical resistance is reserved for randori practice where you can test your, and your opponents, technique, and avoidance at different levels.
Presence. Be active and aware at all times as uke. Expect the unexpected and flow quickly with it.
Progressive Confidence. Confidence comes with repeated practice. As soon as you feel comfortable with a kneeling forward ukemi, progress to a standing ukemi and so on. There is no substitute for mat time.
** Technical Variation: In Shodokan/Tomiki Aikido and Judo; "Rolling Breakfalls" are generally practiced keeping the rear leg straight (slightly bent) so that the knee joint and ankle does not touch the mat, rolling instead onto the edge of the foot. Practitioners of Parkour adopt this principle for practical reasons when rolling on hard surfaces. (To avoid potential contact injuries at these vulnerable points ). It is common practice however within other schools of Aikido to fold the rear leg at the knee and roll onto the leg across the tibia/fibula (from knee to ankle) for a more comfortable mat supported breakfall.