It was a sort of enclosed worm gear, both of us used these on training bikes in the 1960's. No perceptible drag, had one on our tandem in the 60's, but the 40 mph calibration was not enough for slight downhill.
This was a way of showing Mrs.K how to keep up the speed on inclines, which for TT's is more important than trying to catch up downhill, if you do the arithmetic.