I find that notch, no matter what setting, will knock out most of the larger gold rings. The idea is that there are more smaller rings in parks that fit under the pulltab range, so using notch to knock out pulltabs will still allow you to get that higher % of gold. As to what the notch does specifically, use the search function, there are others here who have explained it with diagrams and you might understand it better than what I would. But here is my explanation:
The notch function, for all intense and purposes is fixed. You can't move it, but you can widen it or narrow it. Is sits right on top of the pulltab conductive range.
In the disc range, you have: ...iron......foil......5c......pulltab......1c......silver... The notch only functions when you flick the notch switch to "narrow" or "wide". This will also activate the notch turn knob to be used for customising notch.
With notch functioning, you have: ...iron......foil......5c....|...notch zone...|....1c......silver...
__________________________...Start of notch zone ^ __________.. ^End of notch zone. In between these zones, everything is notched out.
The "notch width" knob will move the start of the notch zone left or right, indicated in red: ...iron......foil......5c....|notch zone...|....1c......silver...
The "narrow"/"wide" notch switch will move the end of the notch zone accordingly, indicated green: ...iron......foil......5c....|...notch zone|....1c......silver...
This way, you set it up however you want to set it up. You might want to notch out more than just the pulltab, so you set it wider on whichever side...or go narrower to let more targets through. When you have the notch switch in the off position, the "notch width" knob has a different functionality. It shifts the position of the lower two tones. There are too many variables to mention, you set this up however you want it to.
You can also use the tones as a guide to what you want to dig, especially if what you're looking for only falls into one of the range of tones. Most Silver for example, will only fall in the highest tone range. This way, you can keep the disc down in order to get more depth, and only listen for the highest tone if you're only looking for silver. That's just a simple example. It's up to you how you want to advance yourself with it.