GB Notching is a funny duck at times - I know it works, and works well...so I will try to explain it as I understand it...criticisms and verification are always welcome!
So in Heinrich's video where he is adjusting the Deus to find the deeply buried (planted) coin, he notes that the Deus in these hotter programs becomes more sensitive to shock, temperature flux, and changing GB readings. This is true with ANY detector, and if you are after those smaller and deeper targets you will need the POWER and STABILITY to see those deeper and smaller targets without having the signals being confused with background chatter.
The way I understand GB Notching is when you "Notch Out" an area from say, 85-90; you are telling the software to REJECT ground readings at "85" and above while ACCEPTING ground readings below "85". What this does is reduce chatter that comes from very localized "patches" of ground that have a local GB reading of "85" or higher. Without the GB Notching, sometimes mineral and iron-laden "patches" of ground are more electrically conductive than the "average" ground in that area. Black sand and other iron compounds conduct electricity slightly better than non-mineralized "patches" of ground - but on the other hand black sand areas can also contain higher concentrations of gold (silver and copper)...so having too much ground notch can also be counterproductive.
Deep iron objects corrode into many of the same compounds found in black sand, which can alter the EM detection field in a way that reduces EM penetration...which could mask a deep non-ferrous item if it's closely associated with the iron object.
With the Deus, most people (who are not nugget hunters) want stable operation with good depth, and less "deep iron bleedthrough" so that one doesn't dig deep iron all day long. Notching the first 5-7 points (83-90 or 85-90) gives stable operation with good depth and separation abilities, and these settings will work for 90% of the dirt out there. Personally I've found 8-10" deep coins ranging from silver dimes to nickels in various types of soil ranging from loamy to rocky to sandy and all the in-betweens.
One of the reasons the GB Notch works so well for "concussive GB changes" like bumping into tree roots, stubble, etc is that the Deus is a motion-based VLF machine and the software is looking primarily for CHANGES in ground readings. A coin in the ground is a MAJOR change in the EM field and thus, we detect it. Not sure how many "samples per second" the software is capable of, but when you bump the coil into a tree root or similar it takes the software a split-second to rectify the changes in ground readings vs a sudden stop, which the software interprets as a CHANGE before it samples more ground in a "rest state". A faster processor and/or smaller EM field would be less sensitive to "concussive changes". What the 85-90 GB Notch setting does is REJECT these concussive changes and promotes a smoother running yet still very sensitive program.
This is all theory of course, I have no scientific anything to back this up, but this is how I understand it....Next time you dig a deep, corroded target remove the said target and scan the dirt around where the target lay and you can sometimes see the GB reading numbers change while scanning the "halo area"