I'm a Swedish writer and aikido instructor, 6 dan Aikikai Shihan, former Vice Chairman of the International Aikido Federation. I've practiced aikido since 1972. I also teach the sword art iaido. Here's my budo bio.
by Stefan Stenudd. The aikibatto sword and staff exercises for aikido students explained, with practical and spiritual aspects of the sword arts, equipment for training, and more. Click the image to see the book at Amazon.
Increase your life energy, by Stefan Stenudd. The life energy qi (also chi or ki), with exercises on how to awaken, increase, and use it. Click the image to see the book at Amazon.
Sankyo in Tantodori Tsuki
The Aikido Technique Sankyo on Tsuki Knife Attack
When taking the knife from the attacker in the sankyo technique, first make sure that you have applied a good sankyo grip on uke's knife hand, twisting it firmly, as shown in the picture above. In tantodori I recommend a stronger application of the sankyo wrist turn, than what is needed in regular aikido training.
Sankyo on different knife attacks.
Also, keep your elbows low, both for additional strength in the sankyo grip, as your body supports it this way, and for protecting your own body.
This position should be attentive. If uke tries, for example, a side kick, you can stop this move and make uke bend over by making a sword cutting move with your sankyo grip - or you could force uke to trip backward in a circle, by turning the wrist additionally toward uke.
Watch the knife carefully - how it is held by uke and in what direction the edge of it is pointing.
In the case of a tsuki attack the knife should normally be positioned as on these pictures, but it is not certain. If the attack had been done from a knife grip where the edge of it was initially upward, it would be in a reverse position here, and removing it would have to be done slightly differently from what is shown below.
The knife here is single-edge, which is more easy to handle than a double edge one. This too must be carefully observed before taking it from uke.
Now it is time to take the knife. Release your hand with the lower grip on uke's fist, and form it into a fork shape, like the letter U, as shown on the picture above. Do not bend your fingers, neither the thumb nor the others. This shape of the hand should be kept until the knife is safely removed from uke's grip.
To avoid the knife falling out of your reach, pinch it a little at the base of your fork grip. If you do this already when you apply the fork hand to the knife, you will be sure to get hold of it whenever uke drops it.
Press the knife up to a horizontal position, then move your hand in a semi-circle toward uke, allowing the backside of the knife blade to press against the bottom of the U, that is the area between the thumb and the index finger.
The semi-circle should be done with uke's hand as the center - fixed at the same spot all through. It is very important not to let uke's arm or hand move, or uke may very well get a better grip on the knife.
Never try to pull the knife away, before completing this semi-circle, even if uke's grip seems to be loosening. The knife will surely break free at the end of this movement, if not before. When doing the semi-circle, turn your whole body the same way, for additional strength and control in the movement.
Apply a good grip on the knife, before moving away from your position or releasing uke's arm.
This technique is almost identical to that of sankyo in the yokomen uchi attack, and so are therefore most of the instructions.
My Aikido Books
Basic Concepts of the Peaceful Martial Art, by Stefan Stenudd. Aikido principles, philosophy, and basic ideas. Click the image to see the book at Amazon.
How to do kogeki, the attack techniques, by Stefan Stenudd. All the attack techniques in aikido explained, and how to do them correctly. Click the image to see the book at Amazon.