yeah, that's the trick.. 
no you need to be strong too yes yes.. but he pushes his body not as much upwards as he is pushing it backwards at the same time as he bends forward at the hip allowing his center of gravity to be closer to his feet, which makes for much more air time to do the three claps. it's worth noting

edit: obviously still very strong and impressive. i can't even do one clap behind my back despite weighing nothing.