The knife-hand low block is a lot like the outer forearm low block. But there are some important differences to remember. The most obvious is the blocking tool itself.
The knife-hand is the area on the side of the hand between the wrist and the main knuckle of the little finger. You can concentrate lot of force into that little area, so this block is meant to break its target as it redirects it sideways.
To make a proper knife-hand, press all four of your fingers together, while keeping some space between your thumb and the rest of your hand. Bend your middle finger slightly, as well as your ring finger if you can. Bend the tip of your thumb toward your palm. You should feel the fleshy part of the pinky-edge of your hand get much firmer.
The preparation is like the outer forearm low block, except the arms cross at rib height (just under the armpit) instead of at shoulder height. This is because this block moves more sideways than the outer forearm version.
Think of knife-hand blocks as switchblades: keep your hand closed until the last instant. At the finishing point the elbow is bent about 15° from vertical. This means that your arm will extend away from your body a little more than it will when doing an outer forearm low block. The distance between your palm and thigh should be about 15 cm. You should be half-facing and your knife-hand should be in line with the centre of your shoulder.
The inner and outer shinbone are the listed targets (try to hit nice and close to the ankle). I suppose there’s no reason you couldn’t use this to block an arm as well. As I said above, unlike the outer forearm block the knife-hand low block moves somewhat sideways across the body to hit the proper targets. But just like the outer forearm low block, move your arm in a straight line from chamber to finish, instead of in an arc.
As with the outer forearm low block, the hand and arm are rotated on a slight angle. Again, the angle seems to roughly mimic the angle that your hand had to travel to get to the finishing position.
As you can see, the differences between the knife-hand block and outer forearm low blocks are small, but important. So now you know how to make yours even better!