So I installed the NW coolant separation and (without separating the system yet) after warmup the coolant indicator on the car is dead center between high and low. It previously was at maybe 3/8 - 7/16, but definitely below halfway.

So I looked around and came upon this (old) thread. I did not purge the system, since I wasn't sure how to.

If I understand correctly, I pinch (put a clamp on slightly) the T line just to the right of the T in the diagram shown in the second post? If I let the car idle while I do this, how long should it take, and how would I know when it's done?

I also am not sure I really understand why this works?