This glitch allows you to kill Koin off-screen. It works in both versions of DKC3, and it also sorta works in DKL3 as well (but not quite as well). In DKC3, take a steel barrel and and throw it to Koin when he's off-screen. Because he's off-screen, the barrel won't hit him, even though the barrel is still present. You can let the barrel bounce off a wall and let it bounce back. At this point, you want to move towards Koin so that the barrel will hit Koin right when he appears. If the barrel touches Koin right when he appears, Koin will be defeated and you can get the DK coin. Also, sometimes you don't even need to make the barrel bounce back either.
Like I said, this also kinda works in DKL3, but it's a little different. Here, Koin has to be WAY off the screen for this to work (a little less than a screen's length away), and when the barrel touches Koin when the game recognizes that Koin is there, he'll just deflect it. So you want to make Koin appear BEFORE the barrel hits him.
Here are some videos that show this trick:
GBA version:
http://www.youtube.com/watch?v=9h-FEZmIV4M
http://www.youtube.com/watch?v=lzKZL51t3hE (variation of the trick that involves Kabooms)
SNES version:
http://www.youtube.com/watch?v=rGll4aepq6k (see from 6:21 to 6:34)
http://www.youtube.com/watch?v=f_X-Ige6tTA (see from 2:48 to 2:58)
DKL3:
http://www.youtube.com/watch?v=oSnLUE3Rg_c
http://www.youtube.com/watch?v=Brr7xqYJoWY (see from 1:51 to 1:55)