I grew up with a brother four years my junior. In that distant politically incorrect era our toys included a pair of plastic knives. I can recall several games where my brother had a knife and I grabbed his hand to wrestle it from him, just like the heroes in the old movies I was growing up watching. At his young age my brother did not understand this is what heroes were supposed to do. He would reach over with his other hand, take the knife and then “stab” me.
Fast-forward forty years or so and I see knife-defence courses telling Police officers to seize the knife arm in both hands. Whether or not this is the optimum tactic and how easily or not it can be achieved will not be debated here. By design or chance you may indeed end up holding a knife arm with both hands. If a three year old can work out the tactic of changing hands (“foisting”) so can your assailant so you had better do something before he does.
What you can do exactly will depend on the relative positions of your hands and his arm. Are your thumbs on the radial or ulna side of this arm, the inside or the outer? Is this arm held high or low? Are you standing or on the ground? You do not have time to start changing grips so must execute any technique from the hold you have. Our objective is to take his knife away from the reach of this free hand and preferably put him at a further disadvantage.
There are essentially three directions we can take his arm: up, down, and out.
Downward involves swinging the arm down in a vertical half-circle, twisting it, moving your body and changing your grip if needed. The arm will finish in either a straight arm lock or a bent arm hammer-lock. These are powerful locks that can cause dislocations and breaks and may bend the attacker forward.
Taking the arm out involves moving it horizontally out. This may or may not involve twisting the arm along its long axis. This is powered by rotation of your waist and moving to his outside gate, taking the knife well away from his other hand. If the leverage from his arm can be used to throw him, do so. If possible, keep control of his arm so you can apply pressure to it with your leg
Taking the arm upwards is a little more involved and is easier from certain relative positions than others. Use this method if it is quicker or easier than the other options. The target position is to have the attacker’s knife arm bent at the elbow with his hand behind his neck. There are several ways to do this. If his hand was already raised we circle around him to take the knife back and behind him. We continue the pressure to lean him backwards to destroy his balance and use or leg or body to help him fall. It is also possible to apply this lock by making an outward turn, turning away from him and taking his hand over your head and on behind his shoulder blades. This action is reminiscent of a judo throw and can be used to throw the attacker down. It is possible that the attacker could reach his knife hand with his other hand so like the other moves you must apply this with vigour to take him down as quickly as possible.