Iron mask is a mode all to itself. Push the button once and it is in the set mode where you will set it to what you want, push it again and it is in the iron mask mode we hunt in after we set it. now if you push it again you are out of the iron mask mode and into your disc pattern. When in the disc pattern the -8 you set the iron mask is not used in the disc mode.
The reason many of us use the iron mask is that we are more in a open screen so we will hear most everything other than what we set the iron mask at. With hearing more we will also hear the coins that are close to iron nails and other smaller items as like the name says,the iron dont mask the coins as much. with this we also notice more depth as there isnt as many nulls and it can see futher in the ground, but you will hear more signals too and with expereince you will be able to tell the good from the bad.
I will switch back and forth between my iron mask and my disc pattern at times to check some signals and when in iron mask you just have to hit the button once and you are in the disc pattern and when you want to go back to iron mask you hit it twice.
Good luck
Rick