Skateboarders doing their thing (very well) at the Ganges community skatepark built in 2004.
Capturing fast motion at the peak moment requires lots of practice, a camera capable of very fast focus and subject tracking, and one able to shoot many frames per second. A fast lens allows a sharp, shallow focus on the subject while the spectators in the background are out of focus.
Technical: Canon EOS 1D Mark II , ISO 100, 1/500 sec, f/3.2, exposure compensation +1.0 stop, 135mm f/2.0 lens
See also: Motocross post for more examples of shallow focus.