In the early patterns, the low block is often done with the outer forearm. The outer forearm is the bony area on the pinkie-finger side of your arm, just above your wrist. It covers one third of your forearm, starting at the wrist (but not the joint itself). This is a relatively big, heavy area on your arm and so the block is designed to hit its target with blunt force.
All low blocks (not just this technique in particular) are meant to block attacks directed at the lower abdomen down. Virtually all low blocks should make contact with the attacking hand or foot at the same height as the target that the attacker is trying to hit. In your patterns, the hypothetical attacker is aiming for the pelvic area, so that’s where the blocking tool ends up.
When preparing for the block, the arms cross at shoulder height. The blocking arm reaches across the body and crosses on top of the other arm. The backs of the wrists touch one another. A lot of people keep their hands too close to their bodies. Don’t do this. Reach your arms out. Give them room to move.
Because this movement involves a twisting motion, the preparation should involve an extra quarter-turn. In other words, since you end up half-facing the target, you prepare by momentarily twisting your body to roughly three-quarters facing.
At the finishing point of the block, the elbow is bent 25° from a straight position. If you are in a walking stance, the distance between your under-forearm and thigh should be about 20 cm, and your forearm should be parallel to your thigh. (This actually seems to be the case when doing the block in most stances, not just a walking stance.) You should be half-facing the target at the moment of impact (i.e. turn your body to about a 45° angle).
Since you’re half-facing, this is considered a side block. This means that the blocking tool (the outer forearm) should be in line with the centre of the shoulder on your blocking arm. As with all blocks, at the moment the block is completed the shoulder of the blocking arm should be lowered slightly.
According to the Encyclopedia, the front of your opponent’s shin or the back of their punching arm are the typical targets for the block. Because you are coming down primarily on top of the target, and not deflecting it to the side, make sure not to swing your arm sideways across your body. However, you aren’t coming straight down, either. Rather, move your arm in a straight line on an angle from chamber to finish.
With that said, if you are performing the block in a sitting- or X-stance, because you are moving your arm more sideways across your body, the outer or inner tibia/forearm can also be targets.
Because you are usually hitting the target at a downward angle, your fist and forearm have to be rotated to the proper angle as well. Your fist is not pointing straight up and down, and it isn’t sideways, either. I can’t find an exact number for the angle, but it seems to follow the same angle as the path your arm just travelled. Imagine that instead of blocking something, you’re about to smash yourself in the thigh with your sidefist (while following the same path as the block). That should give you a good idea of how your fist and forearm should be turned.
That covers all the major points, but as with anything there are always a host of fine details. If you have questions or anything to add, feel free to leave a comment below.