Smart Fingers Turn Your Hands into Rulers

This is almost it. They missed it, by just a little. There is no way you'll persuade people to use these weird things. However, disguised as a pair of rings and a a wristwatch, I'd want!

What's the technology behind the measurement? It would be nice to duplicate it using two cellphones.

I thought "smart fingers" would be another "measure your own hands" hack, perhaps with improvements and tips of some sort.

Who exactly would buy these things? They're clearly not designed as a precision measuring tool, but how often do casual users need to measure things? Enough to give up a socket or two to the wall-wart this thing will require?

My hands are already rulers:

Distance from tip of thumb to tip of pinky = 9 inches when fully spread.

Distance from base of thumb to tip of index finger = 6 inches when thumb is at right angle. Combine two hands to measure a foot.

Max spread between second and third fingers = 3 inches.