This cheat may not be familiar to non GBA players, I'll explain.
The ROCKARD cheat enables the hard mode in the game. No DK Barrels neither Midway Barrels, and both kongs appear in the next level entered if only one was available. Something also included in the SNES version.
Now, the GBA version also included many minigames, one of them being Diddy's Dash.
Diddy's Dash is a minigame where you get to play every level in the game only with Diddy and try to achieve the best time by either using shortcuts, like bonus entrances, or defeating multiple enemies to stop the clock.
But if you apply this cheat and then return to the main menu and enter Diddy's Dash, the cheat would still be in the game's memory and would apply it, leading to many glitches and the posibility of using Dixie.
1.- Enter Pirate Panic and inmediatily throw Dixie or Diddy to the left, go through the level and enter the bonus barrel, you would inmediatily be blasted like normal, but then the game would start to lag horribly, and Diddy and the enemies would fall off-screen. You then would be able to move either left or right with a glitched camera, but doing so to the left would usually lead to a game crash by either a blank screen (usually in emulators) or a high-pitched electronic sound with a static screen (most common in the original GBA). And for some reason, doing this glitch with the European version (tried the ROM) will not lag the game, but can also crash it.
2.- Go to Rattle Battle and team-up, enter Rattly's cabin. The particularity of Diddy's Dash is that if you enter special animal buddies rooms you will not have to enter its barrel, the game will automatically transform you. If you do this you will emerge as Rattly but with the team-ed up kong sprite. You can't do anything with him/her until you jump on an enemy. Rattly would freeze and you would gain the active kong's teamed-up physics. You can now throw the inactive kong. but beware, if you jump on an emeny Rattly's physics will be reseted. You can even complete the level with the glitched Rattly by throwing the kong to the first high platform or fly with Dixie!
And many more...
Here are some YT videos I found a while ago:
http://mx.youtube.com/watch?v=3shsIKKZDzk
http://mx.youtube.com/watch?v=n_N4Tt4dX4Y
They don't particularly show the ROCKARD glitches, but they do show it.