Mike Wehner:
Once the tissue is dead — which, in the case of someone chopping your finger off without your consent, should happen within a matter of minutes — two things will happen. First, the finger will lose all electrical charge and will fail to even activate the sensor, and secondly, if by some chance the sensor could be artificially activated, the RF reader that is searching for a print will find no living tissue and fail, leaving the device locked.
I’ve had several people ask me about this topic. Usually, it goes like this:
“Won’t thieves just steal my phone and then cut off my thumb?!”
I wonder how Apple’s engineers tested this while developing the iPhone 5s. I know internships can be rough, but still…