This assumes e.g. that I can extend my ring finger without also extending the adjacent middle finger. ;)

Rather than extending extending and retracting fingers, you can achieve this by placing your hands on a table and lifting a finger off the table in order to indicate a 1 on that digit.

http://en.wikipedia.org/wiki/Finger_binary

It just takes practice! https://vimeo.com/89914687

2^8 = 256

Haha, touche..