Re: Discharge dyeing, I learned last year to use diswasher gel with bleach to stamp or screen a design on a pre-dyed fabric. Here's an example:

In this instance, I dyed fabric using Low Water Immersion, then used a screen design I made and squeegeed through the diswasher gel. As it dries, it bleaches out the color from the fabric.
Here's another example, where I applied the gel using a stamp I made on a commercially dyed shirt:

As for 'overdyeing', it is simply where you dye something, usually with LWI technique, then after laundering, you apply one or more other dye colors, instead of using one dye bath and several colors.
Hope that helps!
Judy