From www.citeworld.com/security/22399/iphone-fingerprint-scanner-better-biometrics
With the new sensors you don’t have to move your finger, just press it against the reader. And like the sensor in the iPhone 5S, the sensors that will be in laptops and keyboards and other phones can detect the ridge and valley pattern of your fingerprint not from the layer of dead skin on the outside of your finger (which a fake finger can easily replicate), but from the living layer of skin under the surface of your finger, using an RF signal. That only works on a live finger; not one that’s been severed from your body.
This will protect you from thieves trying to chop off your finger when they mug you for your phone (assuming they’re tech-literate thieves, of course), as well as from people with fake fingers using the fingerprint they lifted from your phone screen.
Well it won’t really protect you from thieves chopping off your finger, it will just give them less reason to, I guess.