From what I understand it tests the minimum retraction distance you need to avoid stringing. The lower you get the less retraction you need. For example, for me usually it stops stringing around 0.4mm retraction (that’s 4mm measured from the hot plate), but found that in real conditions the default 0.6mm works better. I don’t find this test too useful, for me it fails to demonstrate the spectrum between too little and too much retraction, a feature I appreciate in the pressure advance tower. Apparently the moment it stops stringing, anything after that won’t show you anything new and it’s best to stop the print. Either that or I fail to notice any defects when the retraction is relatively high.