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 a simultaneous attack and defense, it is also a Kūsankū movement.
Mountain punches are used to defend and counter against circular strikes, like haymakers and club swings. Step same-side foot towards the opponent, or 7-3 to the inside, 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, bending your elbow slightly upward; your arm will simultaneously jam the opponent's attack, and shield your head. Simultaneously, strike the opponent's solar plexus with an opposite-side uppercut.
[video of mountain punch from the font and side.]