I know that Action Replay/Gameshark is a cheating utility. But what is Swap Magic?
Whereas AR/GS are cheat utilities that can be misused to load a backup disc, Swap Magic is a set of discs (1 DVD/1 CD) specifically engineered for booting backup discs. They are fast, simple, and effective you use. Simply start the PS2 with Swap Magic and wait for the loading screen to come up, then swap with your backup disc using a slide card or similar technique. Swap Magic also has the added benefit of changing the color system of the PS2 on load, for playing imported games (PAL/NTSC).