Let me take a shot at it. A pulley and a diode can both make a machine that causes vector reversal.
1) I can pull DOWN with a pulley and lift a weight UP.
2) I can put a NEGATIVE half sine wave into a full wave rectifier and get a POSITIVE half sine wave out.
Taken a step further...
1) I can get current flow through a forward biased diode (electron motion). But I can't get current through a reverse biased diode (no electron motion). (Unless it's a Zener, they cheat.)
2) I can pull on a pulley rope and raise a load (motion) above a floor. But I can't push on the pulley rope and lower the load past the floor 9no motion).
How's that?
