This technique is used against a straight punch.
When your attacker strikes, block it with a basic inside forearm block. Step forward with your right foot so you are sideways on to your attacker and strike with a hammer-fist to their bicep.
Your feet should be placed as shown in this close-up.
Strike with an elbow or back fist to your attackers ribs.
Bring your right arm under your attackers arm as shown and turn into them.
Bend your knees and pull your attacker onto your back.
Straighten your legs to lift your attacker off the floor.
Now simply roll your attacker off your back by twisting your upper body slightly. You can now finish with a strike or maybe an arm-lock.