Hittsui uke (literally: “knee-hammer block”), the knee block is used to defend against attacks directed below the waistline (e.g., groin, knees, feet), because hand-based blocks would require leaning forward to reach those attacks. This leaning would compromise your balance. Additionally, your head would become closer to the opponent, making it easier to hit.
There are two versions of the knee block: passive, and active.
The passive knee block is simplicity itself -- enter a fighting crane stance. That’s it -- that’s all there is; it’s that simple. Your thighs are padded with muscle, which will act as a meat shield to protect your genitals. Additionally, knee blocks are the ultimate defense against leg sweeps, since moving your foot gives your opponent nothing to sweep. This is an extremely simple technique, which is appropriate for Beginners and novice students.
[video of passive knee block, from the front and from the side, fast and slow]
The active knee block is similar. The act of entering fighting crane stance -- pulling your knee up and across the waist to the opposite-side hip -- can swat incoming attacks aside, while setting up a kicking counterattack.
It should be noted that both versions are, in every way, the same technique. Beginners and novice students do not use the active version only because it requires a well-developed sense of timing. If the block comes too early, it becomes one of the passive knee blocks from earlier. If the block comes too late, then you get hit. It is best to err on the side of caution and block early, and as you improve, you can reduce that buffer time until you can actively block. This is critical because exclusively relying on passive knee blocks will really beat up your shins after a while. Many of our beginner, novice, and intermediate students elect to wear shin pads while sparring until they get their timing worked out.
[video of active knee block, from the front and from the side, fast and slow]