Bob,
When the Explorer XS came out 20 years ago the manual made much ado about using a ton of discrimination and "teaching" in good targets. It would have been nice if it worked, but, in real world conditions where there is a ton of targets under the coil at any one time, the great majority of targets don't consistently hit where they are "supposed" to hit. If you are running a lot of discrimination most of the time these hits will cause a null and not a signal. I ran with what the book told me to do with patterns for a week or so, then started realizing that I was missing a ton of good targets. By opening up the screen you no longer got a null. It might not hit in the "supposed" spot every pass over the target, but you still got a noise instead of a null and could look at where the cursor was bouncing.
Pretty soon I had the screen almost totally open, but with iron targets being the predominant trash target the machine was still nulling most of the time. In conductive tones you can't completely open up the screen because nails and iron sound like silver. Many of us then switched to ferrous sounds, where higher conductors give a high tone and iron a low grunt. Basically this keeps the machine from nulling out and lets you listen to everything. I only discriminate out where crown caps hit, lower right of screen in XS speak, because they also give a high tone in ferrous.
It was interesting to watch over the years newer folks come to the forum and reach the same conclusions through their own experimentation; that less discrimination was better. It was a bit controversial at first but I think over half of the XS swingers ended up being ferrous people.
When the Etrac came out the equivalent set up became to be known as TTF- Two Tone Ferrous. As with the XS, you had to be in ferrous to open up the screen. On the Etrac and later models Minelab hugely dumbed down the machine and made pretty much every target other than iron have a ferrous value of 12, that is why it is called Two Tone- Iron grunt or one other sound. On my XS I can tell a nickel from a dime from a Indian head in ferrous sounds and the cursor bounce over the whole screen is also invaluable.
So... In answer to you question. In conductive iron itself sounds like silver, it gives a high tone, if you open up the screen every nail in the world will sound good. So you need to switch to ferrous. Iron falsing- halo effect- actually I'm not sure it is the halo or rusting that causes this, is still a problem in Ferrous mode. Some iron, particularly bent square nails or some round items, can sound much like a coin. I still dig my share after 20 years using the machine, and this seems to affect all the machines I've used equally.
Basically TTF works because in iron laden sites you are not continually detecting in a null. Shallower good targets will often give a signal, but you miss a ton of other targets. In ferrous you will get an audio response.
Chris