The "sound off' off of another metal(coin) coming through the background threshold null is what is sought within that threshold null.
If you were to run All Metal SOUNDS FERROUS so that most iron was represented by a low tone ( no discrimination patterns) on both the E-trac or (the SE in Ironmask and Smartfind) then there there will be NO threshold null.
Two things I'd look for in the SE was where the location and the behavior of the cursor and the possibility of at least a tone, other than the low tone . There have been cases where I only had a bouncing cursor and no tone differential and that to me, meant it was worth investigating what causes that. Some times it was a deep coin, so on that case the null was irrelevant and I could not rely on a coin sounding off or coming through as a tone differential.
Soon as you include a level of discrimination , a null will occur when you pass over an object that is comprised of the material that's been excluded within the realms of a detectors setup..............therefore it will null over that excluded material.
The ability to separate the excluded material(the stuff that is using the null abilities IF you have set the detector to do so) and the material you want to hear(eg: coin) at a depth and with in certain ground conditions is where a detectors abilities mainly lie.
If we're to use absolute AM setting in the E-trac or SE then there is NO nulling of the threshold, not even momentary, when you pass over a desirable or undesirable target. Depends on what you want to find I guess. Not all "desirable" targets are non ferrous.
The E-trac , Explorer's and some other detectors will see through this "appointed" null and depending on a number settings or two, will also determine, in some detectors, what it MAY or most likely MIGHT be that is laying nearest to, under, above or next to the included object/s mass that is causing the null.
Set up the detector so that you can bare with all that background noise of AM sounding of (lower all the volumes and increase the gain) "Tip" sensitivity slightly over the edge(HOT). If the detector was able to see the coin in the other stuff most likely it will..................BUT........................only if it can and there is no "black art" to this fact.
The E-trac can see quite well through this stuff. It's the latest design that takes into account some things that the others do ....but SEEMS to do this better.
I hope this helps.
David E Di