This was an experment with the new Knuckles controller prototype by Valve.
Goal was to make the player use the force in an intuitive way.
The way it works:
Close your hand around the controller,
Pull your fist backwards,
push your fist forward,
quickly release the controller.
If your movement was fast enough, A force push will be done, where the strength is based on the power of your hand movement. It's great fun to knock over boxes and Senates this way!