Yama-tsuki, the mountain punch, is sometimes called a U-punch or C-punch, because of its shape. The mountain punch is a special case of a double punch, and since it is simultaneously and attack and defense, it is also a Kūsankū movement.
Mountain punches are used to defend and counter against circular strikes, like haymaker punches and club swings. Step your same-side foot in towards the opponent to get inside of their arm, so their circular attack wraps around you instead of hitting you.
[video which demonstrates this from the font and side.]
Punch the opponent's face with your same-side hand; your arm will jam the opponent's attack, and shield your head. Simultaneously, use your opposite-side fist to deliver an uppercut to the opponent's solar plexus.
[video of mountain punch from the font and side.]