Yonkyo (四教)
Yon: four.
KyoKyo: form, model, teaching, principle.
Fourth lesson is an immobilization caused by pressure exerted near the ulnar nerve, about 4 fingers from the wrist.
Uke's forearm is controlled as if holding a bokken, closing the fingers firmly, and pressing with the base of the knuckle of the index finger on the inside of his forearm, producing a painful sensation.
Less experienced aikidokas may have trouble finding the right place in to apply pressure, and may be unsure how to gain control over uke's forearm.
The grip is very similar to holding a bokken, with a slight inward rotation of the hands, and it is the base of knuckle of the index finger of the front hand that is exerting pressure on the inside of the forearm.