Downward-fist blocks are chambered by bringing the blocking hand up by the opposite-side ear, with the palm facing inward. Hold the opposite-side hand open over the exposed floating ribs on the blocking side, as in the picture below.
[front-view photo of gedan barai chamber]
Pull the elbow of the blocking arm down diagonally as far as possible, then thrust the fist straight downward. Do not lock the elbow -- instead, keep it slightly bent to draw upon the relaxed tension of the Unbendable Arm technique. The opposite-side hand pulls across the abdomen into a chambered position, setting up a counterattack.
[video of gedan barai, from the front and from the side, fast and slow]
This must be practiced slowly until it becomes one fluid motion, then that motion is gradually sped up. Slow is smooth; smooth is fast. It is tempting to be lazy, and to just swing one’s arm in a large downward arc, but there is no power behind such a block; it would be no different than trying to smack an attack aside with a spaghetti noodle.
[video of a swinging gedan barai, from the side, failing hard]
When performing downward-fist blocks, do not cross the arms in an X-shape across your chest. If you do this, an opponent can trap both pushing on the outside arm to trap them both.
[front-view photos of improper gedan barai, and trapping the arms. ]
Do not use downward-fist block to defend anything below the waistline. These attacks are too low for downward-fist blocks; they can only work by leaning forward, compromising one’s stance and stability. In addition, leaning involves additional and unnecessary energy expenditure to right oneself. Instead, attacks to the groin or legs should be deflected with knee blocks. Leaning from a downward-fist block just accentuates one’s undefended head, making it much easier to reach and abuse. Only use downward fist blocks to protect the abdomen.
[side-view photo of leaning as a result of blocking too low, and being pulled off-balance ]