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, either leading to their incapacitation, or to the realization that attempting to hurt you is counterproductive.
Like all blocks, punching block is the most effective when coupled with an evasion, usually a 7-3. Simultaneously use your opposite-side hand to throw a hook punch to the opponent’s arm. Ideally, this will hit the groove between the biceps and the triceps, as that will cause a painful nerve strike.
[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 to block a block! The unexpected pain will act as an atemi, and this combined with the shock of the impact can knock an opponent off-balance, for an easy follow-up.
Since punching block requires the student to be extremely good at both evasions and hook punches, it is reserved for advanced students.