The entire internet seems to be ignorant of this one, and no one is willing to help... Me
to the rescue!

In Sky Sands, there is a cave with sand falling down from the ceiling. These are "the
quicksands." What the game doesn't say is that you can need to enter these streams of sand
EXACTLY 3 times. It doesn't matter which ones you use, so long as throughout the entire
race you only p*** through one 3 times.

You can easily complete this entry in the Checklist in a 1-lap race. Hope this helps.