Sometimes people rappel without a backup prussik, such as when canyoning and sometimes an auto block needs to be adjusted. In either of these situations, is there a way to stop yourself in the middle of the rappel?
The simple solution is to take the rope (both strands if rappelling on a doubled rope) below the device and wrap the several times around your upper leg and hold your upper leg perpendicular to the ground (this keeps the rope from slipping off).
It's not particularly comfortable and you don't want to do it for an extended amount of time, but as a stopgap measure it works quite well.
To getting going again, point your leg towards the ground and unwrap the rope.
I'm not sure I'd fancy trusting my life to wrapping the rope around my leg. The conventional way to escape a belay is to tie it off, see here
For extra safety you can clip the figure eight onto your harness using a carabiner.