Well this is where I'd have to disagree a little. The pinch test could certainly be done at full throttle and is a way of letting someone who's unfamiliar with tuning sort of sneak up on the correct full throttle tune. But it's far more useful for setting the idle mix because you have no way of knowing just from revs if it's set correctly or not.

At idle, just pinch the line and listen to what happens. If the engine loses revs immediately then it's too lean. If the revs stay constant for a few seconds and then gradually rise a bit before starting to drop off then you're too rich. What you want is a small rise for just a moment when you pinch the line. This is particularly critical with an inverted engine.

The reason all this happens at idle is if the mixture is too rich then excess fuel collects inside the crankcase. When you pinch the line the engine starts using up all this collected fuel until there's just enough left to make a perfect mixture (the point of max idle revs) then it begins to run out of fuel and the revs die off.

If you're running too rich and you open the throttle, the added airflow through the crankcase picks up all the excess fuel and throws it into the cylinder. Then it's either too rich to run or may cool the plug below the temp it can fire the mixture. Either way, the engine stops