Punch... the punching bag. Use the number keys (1, 2, and 3) to... well... punch. Don't start punching until the "animation" has finished.
Give that bag a good K.O! Punching bags have the added bonus of the fact that when they're knocked down, they stay down for the full count of ten. They are also surprisingly hard to defeat. Have you ever seen one just staring you down?! Credits to Newgrounds.com for the awesome music.