This technique is used when your attacker steps forward with a straight punch.
When your attacker throws the punch and steps foward, block the strike with a basic inside forearm block and step to the right with your right foot. You should be facing the side of your attackers head.
Place the heel of your left foot next to your attackers leading foot and brace their ankle with your foot. This will be used to prevent them from resisting the throw by stepping forward.
Place your right hand under your attackers left arm-pit. This hand will be used to throw your attacker.
To finish simply push to the left with your right hand. Since their foot is braced, your attacker will be thrown. It is important to perform this throw very fast to use your attackers momentum to throw them.