I'm confused. Maybe you're talking here about Machine B instead of A? Where we can get 3X + δ with Machine A? The key part for my understanding here is how exactly Machine B work backwords? If I image myself simple pulley with two weights we can't run it backwards, it only can lower one weight...
I started reading "Feyman Lectures on physics" and stuck on his explanation about reversible and irreversible machines. I've tried to read other answers to this question, but I couldn't get the point. Here is the first thing:
"If, when we have lifted and lowered a lot of weights and restored...