If you stop the timer/hit/catch the flagpole when the 1's digit of the timer is a 1, 3, or 6(doesn't have to be specifically 003, 006, or 001, just have the 1's digit be 1 of those #s), you'll get fireworks corresponding to that number. It's not too hard to accomplish this at all, really. Just practice, & you'll eventually get it, sooner, rather than later. Standing at the top of the stair case, on the left edge, wait until the timer hits the 4th second ahead of the number you choose, including the number itself, to start running. As an ex. when you want 6, try to start running from the position I mentioned, & show, when the timer is on digit 9. So, it can also be looked at by saying w/e second you want, just start running at w/e number you chose + 3. Depending on where you stand makes the difference. However, it is also possible to alter your jump mid-jump, so you can sometimes start running early. Altering your mid-jump is a good way to correct for any timing errors you might make.
As for me, you can see that I made a little jolt in movement during gameplay. This was mostly due to me using the keyboard. I don't have a controller anymore(as of now 9-22-09), so for now I must use the keyboard. Needless to say, it's akward, & makes errors visible in my vids, if I'm not careful.
& before anyone asks, b/c the ver. of this on the Wii VC is the actual rom of the game(same for the Game Boy Advance ver of the game), this trick is completely doable in the VC vers.