Discharge ink is also a water based ink but it has an activator added to it to release the dye in the shirt. This activator, or discharging agent, brings the shirt back to its natural bone color.
Pigments can then be added to the ink to essentially re-dye the fabric to a new color. Without this discharging agent, water based inks would soak into the already dyed fibers and not be able to be seen.
To put it into perspective, when printing on paper you’re printing on a white substrate. With fabric that is already dyed, you have to bring it back to a white state so that you can lay colors on it.
In the picture to the right for The Great PNW, we used straight discharge ink with no dye colors. This removed the black dye from the fabric and is a great example of what the natural fibers of a shirt look like once discharged. We love this vintage looking shirt! Once washed, you cannot even feel the print.