Uniracers is kind of odd. You have to take your living unicycle through a series of races and stunt tracks because that's apparently what living unicycles do.
During the stunt tracks, you get points for doing a variety of tricks, but there are a couple that aren't listed in the manual anywhere, the so-called 'Mega' tricks.
The first one is the Head Bounce. While standing still, jump up, press and hold the X button so that you bounce a couple of times
Then let go. Boom! Head Bounce
The second one is a little tougher to do. You have to get some significant air, then press the X button, but release it when you're halfway turned
Then press X again to turn back, which will give you a Tabletop.
Now, you might notice that on your scoring screen, there's a spot for four Mega tricks, even though I've only shown you two.
And I can't find the article to source this, but the reason is that there's four spots for tricks, and only two tricks is because otherwise the scoring screen would have looked funny, like there was something missing.
So don't go trying to find tricks that don't exist, mmkay?