To put it simply, the item glitch is when you build a "bounces combo" that reaches 199 or more bounces on enemies (or on a bouncy box, the one with an arrow, which is used to raise the bounces counter faster). Starting from that point, you start getting crystals if you keep bouncing on enemies, thus allowing you to skip all levels and head straight to defeating bosses, which makes you save an insane amount of time over a "normal" run in which you collect crystals the way it was intended to be by the developer. Watch an any% run to get an idea of what I'm talking about. As you've probably seen, with this glitch you can basically complete adventure mode in over 20 minutes less than a No Item Glitch run.

Yep, that's all there is to it. Everything except the item glitch is fair game.