Attach this to your wrist, and the machine will be powered off when the USB drive is removed from its port.
You mean attach a cord to the USB thumb drive, tie the other end to your wrist, and insert the thumb drive into your computer before using it?
Seems like a hassle. The cord would have to be pretty short for this to work. It might be ok for temporary sessions on a laptop at the public library, but not for daily use with your home desktop (which is likely not on your desk but on the floor).
Someone should make a wireless version. Using a USB wireless mouse with those little snub receivers you plug into the USB port could work.
You leave the snub plugged into the computer. When the mouse is turned on, USB state changes and you have a live USB human interface. Then you type in your disk encryption key and use the computer. If the mouse is turned off OR if the mouse goes out of range of the receiver, USB state changes and computer shuts down. Now you just need to pull the guts out of the mouse and put it on a fashionable wristband or whatever.