On the way to the Wheaton show today, I stopped at Berland's House of Tools to take a picture of the SawStop mechanism. The electronics package detects when something conductive touches the blade, and within 5 milliseconds, stops the blade by jamming a honeycomb aluminum block into the blade.

You can see how the momentum of the spinning blade plus pulleys, belt, and motor has been absorbed by the deformation of the aluminum structure, plus the several significant chips that the blade has torn from it.

There is a great slow-motion video at www.sawstop.com - check it out!