Such an interrupt provides a mechanism for graphics registers to be changed mid-frame, so they have different values above and below the interrupt point. This allows a single-color object to have multiple, horizontal color bands, for example, or for a sprite to be repositioned to give the illusion that there are more sprites than a system supports. The limitation is that changes only affect the portion of the display below the interrupt. They don't allow more colors or more sprites on a single scan line.