Without resisting your opponent’s strength, but stretching it out and leading it in a circular movement – – this is Tenkan.
Uke: Grasps Nage‘s left wrist with right hand.
Nage: [Tai no henko] as in Exercise No. 8 (Photo 72). [That is:] without losing momentum, he pours forth Ki through [the] fingertips of his left hand, making the one point as the center, and turns to the right. Nage‘s throat must turn and face the direction in which the body moves. The mind also must move in the direction of the body’s movement. If the mind remains behind even for an instant, Uke will not follow Nage‘s lead.
Nage then chooses the moment when he will lower his hips and his left arm, and waits for Uke to catch up (Photo 73). As Uke does so, Nage raises his [koshi], slides his left arm, palm up, to Uke‘s neck, and steps in with his left foot to Uke‘s rear, lowers his [koshi] and his left arm again and causes Uke to fall backward (Photo 74).
In this technique, if necessary, Nage can strike Uke‘s chest or abdomen.
Photo 72: ….If [nage] will concentrate on the one point, pour forth Ki from the fingertips of his left hand and thrust out the back of his hand, that left hand will be immovable.
If, when turning, Nage turns too quickly, the timing will be off and Uke will not be able to follow Nage‘s lead.
It the Ki that is poured forth from the fingertips of the left hand were turned always as though describing a large circle, Uke will follow Nage‘s lead very easily. At this time, if Nage looks back, the flow of Ki will stop, the timing will be off and Uke will not be able to follow Nage‘s lead….
Photo 73: When Nage lowers his hips, and does so while keeping his wrist bent fingertips pointing upward, Uke will be sucked into lowering his body too.
Photo 74: If Nage tries to throw Uke, using only his hands, Uke is not likely to oblige him. But it will be easy for Nage to do so if he lowers his [koshi] and throws him.
– – – Koichi Tohei Shihan, “Supervised by Morihei Uyeshiba [O’Sensei]” Aikido the Arts of Self-Defense, pp. 100-101