The Pomeg glitch is basically exploiting a really dumb oversight by the people who coded Emerald. Here's how to do it:
1. Get a decently high leveled Pokemon with some HP EVs. Don't use something stupid like Shedinja.
2. Get that Pokemon down to 1 HP.
3. Use a Pomeg Berry on it.
Here's what will happen. When a Pomeg Berry is used on a Pokemon, it subtracts the 10 HP EVs and sees how much health it took away. It then subtracts this from max HP and current HP. The trick is to make it reduce HP by two and have a Pokemon with just one HP. This will cause the current HP to roll over to 65535(which the game displays as ?35). There is a ton of odd behavoir when you do this. If you battle with the Pokemon and it takes damage in battle of just about any sort, the game enters what seems to be an infinite loop(it may terminate eventually when the HP reaches legitimate values; that is unknown). Eruption and Water Spout are ridiculous under these conditions; I heard of a Smeargle doing over 90% to a Blissey this way. It really doesn't have any real use in a competitive sense, but there is one little trick. When you use a Full Restore or Max Potion on one of these Pokemon, it will "heal" them to zero HP. They won't faint. I don't recall the specifics, but I know you can use it to confuse the game into sending out eggs and blank spaces in your party. An egg will act like the Pokemon inside except with the egg's pallete(looks like it wasn't fully developed). Eggs can level up and learn new moves, and when they hatch, they'll retain moves. Eggs can evolve, but they will remain eggs and when they hatch they'll already be evolved. When eggs do hatch, they lose all experience gained, any items they are holding, happiness, and EVs(maybe a few other things too). This trick could theoretically be used to hatch a level 5 Agility Metagross; however, it is so arduous to level up an egg that odds are the egg would hatch long before it even reached level 15. The evolution thing was tested with Wurmples, Caterpies, and Weedles if you were wondering. Using the blank space is interesting but useless. It sends out the RS Missingno. No, there is no item multiplication or glitch effects. You are just going to freeze the game.
Sorry I don't remember the exact specifics of the glitch, but as you can see it lets you mess with the little cup. Theoretically, level 5 Meteor Mash Metang is legal within little cup rules. It's like using the Mew glitch to get a level 7 Dragonite for the RBY Pika Cup. If you want to replicate the glitch, it shouldn't be hard to do. Trading it to RS and FRLG does some cool stuff too; it generally wasn't all that well tested and only a few people from gamefaqs really remember much about it.