In chapter four of the part one of the lectures, he mentions:
(..) A machine that we actually use can be, in a sense, almost reversible: that is, if it will lift the weight of three by lowering a weight of one, then it will also lift nearly the weight of one the same amount by lowering the weight of three.
and in the picture (is the fulcrum a bit of placed w.r.t the contex ?)
However, in order to get it actually to work, we must lift a little weight off the left pan. On the other hand, we could lift a one-unit weight by lowering the three-unit weight, if we cheat a little by lifting a little weight off the other pan.
I can't make any sense out of this.