You mean the ghost effect? You know the bar on the right makes it more transparent? Well with the eraser, the lower the bar, the less it erases, so say you want the left side disappearing, so you'd start with the right side on low bar, aand keep moving left, doing less and less on the right. Sorry if this doesn't make sense, I don't think I'm very good at explaining things.

its okay, i get the jist of it.

i miss swapnote :[