It´s a block against an attack on the lower body. Your aim is to deflect an attack to the side projecting your forearm downward and slightly to the side.
Put the palm of the blocking hand at the height of your ear while the counter arm is extended and the fist, palm up, at center-chest. Your blocking hand go down to the hip, then sweep the arm across to the opposite side. At the point of impact, your blocking hand turns over to add a twisting force. The counter hand is pulled back to add power.