I'm not a technical type guy so I'll try to explain simply the best I can. When talking about the iron mask on the GT the filtering occurs when you turn iron mask off. That's why Minelab says you might get a slight loss in depth with iron mask off in the manual and also state that iron mask off will perform better in mineralized ground. Filtering is basically discrimination. Think of the discrimination range as a full circle with small iron being at 6:00 and large coins/silver at 12:00 with ground minerals falling between 1:00 and 5:00. I always thought when Tesoro described the discrimination range of some of their detectors as being ED 120 or ED 180 they were referring to the degrees of the circle of discrimination with the ED 180 giving you a discrimination range through small iron. Opening up the discrimination range to include mineralization will make the detector more sensitive and give better depth in good ground. Some Whites detectors like the XLT gives you access to the full circle of discrimination. If you want to open the discrimination range to improve sensitivity while still discriminating iron you have to open up the upper range between 1:00 and lets say 3:00. The closer you get to 5:00 the more the iron starts to bleed through.This is what the hot rock accept/reject switch on the Whites 5900/6000 does and what the + 95 on the XLT does and what the ground zone does on the QXT. The iron mask switch on the GT acts very similar in that with it on it is more sensitive to mineralization and there is some bleeding through or more sensitivity to iron.