This is a $60,- addition you can put onto a table saw. It detects electrical conductivity and stops the saw as soon as it gets into range – from 5000 RPM to 0 in a millisecond. This means you can shove your finger into the spinning saw without it being harmed at all. Of course, you destroy the device and the saw, but especially at that price it’s  going to save a lot of fingers.

The Tinbasher Sheet Metal Blog.