Hittsui uke (literally: “knee-hammer block”) is used to defend against attacks directed below the waistline (e.g., groin, knees, feet), because hand-based blocks would require leaning in to meet those attacks. This leaning will compromise your balance. Additionally, your head becomes closer to the opponent, making it easier to hit.
There are two versions of the knee block: passive, and active.
The passive version of the 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, and will act as a meat shield to protect your genitals. Additionally, knee blocks are the ultimate defense against leg sweeps, as moving your foot give 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 version 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. The active version is not used by beginners and novice students, only because it requires a fairly well-developed sense of timing to execute. If the block comes too early, it becomes one of the passive knee blocks from earlier. If the block comes too late; you get hit. Err on the side of caution and block early, and as you improve, that buffer time becomes shorter and shorter, until you can actively block. This is critical because passive knee blocks will really start to beat up your shins after a while; this is why many of our beginner, novice, and intermediate students elect to wear shin pads while sparring.
[video of active knee block, from the front and from the side, fast and slow]