I'm not 100% sure, but I believe that's how you fake a shot. Shaking the nunchuk while holding down Z will make the green diamond appear. I haven't thoroughly tested it but you should try doing that (holding Z down) and check the replay to see the fake.
Don't know how useful faking a shot would be though, but I guess it adds a flair to the game.
Muslim Board http://www.gamefaqs.com/boards/gentopic.php?board=563727 --Don't legalize it-- Not worth it.
Yes emmjay101 is right - that fake shot or deke if you like. It really doesn't do a fake shot as much as change direction quickly. Holding that combo down while in offense (Z+swing nunchuck) with high technically skilled players - Ronaldo, Ronaldhino, Kaka etc or anyone with high 80's & 90's+ in dribbling, it will do trade mark moves like the flak flak , sometimes a more controlled fake or even the pirouette.
Its not 100% and more often than not puts you in an off position or you and up losing the ball. This combo has been more effective against Goal Keepers I have found. Even on top player or even in multi-player the computer AI is not the hardest or the most intelligent. About 70% of the time you will dodge past the keeper into an empty net. Whereas about 50% of the time against a player you will just end up losing the ball.