Synthrapol suspends unset dye particles in water so they rinse away instead of settling into fabric. Once a bleed has already occurred, it can require multiple washings in Synthrapol to get it all out. If the bleed has been heat set, it might not all come out.
Synthrapol doesn't work on other types of stains -- only dye bleeds. And it is best used to prevent bleeds, although is often effective also in getting rid of a bleed.