The pinch test works because excess fuel (if running rich) tends to collect inside the crankcase so it acts like a very small fuel tank. This isn't really desireable

Pinching the line, particularly at idle, stops any fuel from going through the carby which allows any collected fuel to begin to go up into the cylinder. As the fuel gets used up it gets closer and closer to being an ideal mixture so the revs begin to rise then when nearly all the fuel is used the revs begin to drop as it leans out.

That's the theory, in practise what you're looking for is a very slight rise in revs when the line is pinched. This means the mixture is just barely on the rich side of being perfect. The rise in revs should be almost immediate after pinching the line and only last for maybe a second. If it's too lean then the revs will start to drop the moment the line is pinched. It's a bit harder to do at full throttle though because any excess gets used up very quickly.