Chūdan uke (literally: “mid-level block”) is the basic defensive maneuver used in most styles of karate, and in most karate-derived martial arts. This technique is sometimes more specifically called chūdan-uchi uke (literally: “mid-level inside block”). Chūdan uke defends against attacks directed between your shoulders and your solar plexus; by definition, chūdan uke excludes defending your head and neck. Other karate styles rely on rising block variants (jōdan uke; “high-level blocks”) to defend their heads. For this reason, Goshin-Jutsu substitutes its cross-body block in lieu of chūdan uke.
Chūdan uke is setup like a cross-body block, so cross your forearms horizontally in front of your chest, at breast level. The blocking arm should be on the bottom, with the top arm pushing down on it, and the bottom arm pushing up on the top arm. This way, the technique will spring into position, rather than being set in place.
To actually throw the technique, pull the top hand to the same-side hip, to set up a counterattack. As mentioned earlier, the bottom hand springs out, snapping into position. For additional speed, do not swing the fist out; pull the elbow in to snap the technique. When all is finished, the upper arm should be pointed at a ~45° angle towards the floor. The forearm and the upper arm make a 90° angle, with the forearm pointing at a ~45° angle towards the ceiling, with the hand clenched in a fist. The video below will demonstrate the correct form:
[video of chudan uke, fast and slow, from the front and side.]
The block makes contact with outside of your radius bone, below the wrist on the thumb-side of your hand, since there are no important arteries, veins, or tendons in that area. The block contacts the bony side of the opponent’s wrist as well, to exert the greatest leverage. While bone-one-bone collisions can be painful to both parties, this only problematic if you forget that karate blocks are meant to be parries.
[Photo of chudan uke blocking point]