When you set the "high speed" needle valve, you're setting the "big" needle valve that is obviously "the needle valve." And you set it with the carb wide open. You normally will start your engine with the throttle set about 1/4 open or wherever you're comfortable and it starts easiest. But to set the high speed, you do need to open the throttle fully so the engine is running wide open when you adjusting the needle valve.

When you have that set, you then are going to take the throttle to idle position to set the low speed needle. So when you do the pinch test, you're actually doing a different test than when the engine is screaming at full revs.

BTW, setting the idle is an entirely different deal than setting the primary needle. A couple of things are different.
One of the first things to do is to let the model sit for a minute with the engine running at idle. Then do the pinch test. But before you tweak the low-speed needle, do a second test. Let it run at idle for a minute and then jam the throttle wide open quickly. What you're looking for is the engine to simply jump from idle to flatout without stumbling or choking. This is actually the point of the whole deal about setting the idle mixture. The pinch test is a good one for either the full throttle test or the idle test, but you can fine tune the idle setting and the low speed needle setting by letting the engine idle for a minute or two and then slamming the throttle open. BTW, when you're testing the idle setting, you don't need to hold the model up, pointing at the sky like you do when testing your full throttle needle setting.

And another minor detail when setting the lowspeed needle. Shut the motor down to do it. It's lots safer that way. BTW.... setting your transmitter to run the engine at a slow idle and also be able to also shut the motor off is another thing you need to do........ chuckle..... yet another intricate detail, right?

I think the confusing thing about the pinch test for you was the minor detail that you do it with the engine flatout for setting the primary needle setting and with the engine at idle for setting the lowspeed needle, right?