The really ingenous part is that the belt attempts to rotate the take up
spool about 10% faster then the other spool. That's what tightens up the
tape if it ever gets slack.
How does that work? All points on the belt move with the same speed, the
belt drives the outside of the 2 spools, so the surface speed of the
spools must be the same. That's the tape speed. I don't see how it can
move the tape faster on the takeup side.
-tony