Tsuki-uke, the punching block, refers to the defensive applications of hook punches. The punching block is hardest of all karate blocks -- you literally attack the opponent’s limb faster than they can attack you.
Like all blocks, punching block is the most effective when its coupled with an evasion, usually a inside 7-3. As you evade, simultaneously throw an opposite-side hook punch to the opponent’s arm. Ideally, this hits the groove between their biceps and the triceps, which will temporarily incapacitate the opponent's arm by striking one of the many major nerves which run through that channel.
[Video of a punching block fast and slow, from the front and side]
Since the opponent is committed to attacking, they are unlikely to block this technique -- rarely does one think of blocking a block! This block ideally also acts as an atemi, due to the unexpected pain, and off-balancing from the shock of the impact.