I agree, there is no real way to remove the dirt from out of the cracks in a clear coat without removing clear coat too. What the eraser does is to sand it's way down through the crack and you stop when the dirt goes away. This exposes the screen art. Keep in mind that screen inks are a little different than clear coats, they are softer, you need to protect the screen work from any ball wear. The reason for clear is that it does this. If it was the other way around and the ink was harder, you would clear the plywood to seal it and then screen it.
I've read about people that have kept going with the eraser and they've removed artwork too, there were posts about this on RGP when ME's first started being used. This shouldn't happen if all the eraser did was remove dirt.
Steve
Quoted from Baiter:Magic eraser is a fine grit sandpaper. It will sand off the top layer of a playfield depending upon how much rubbing you do... which is why it is successful removing swirl marks. IMO it should not be used in regular cleanings.