It's confusing because the drawing omits a pulley attached to shaft "a". Shaft "b" is hollow, shaft "a" turns inside shaft "b". As drawn, shaft "b" will rotate and turn bevel gear "B". If the belt were moved to the [undrawn] pulley on shaft "a", bevel gear "A" will rotate, causing "C" to turn in the opposite direction. The topmost pulley presumably slides left or right to engage the different bottom pulleys.