A post-challenge spin is performed by holding left or right while crossing the finish line in a level. The boat will hop in the air and spin several times.
Edit: As BryanRPh has pointed out each spin counts towards the achievement, getting 4-5 spins per level. You should be able to get this in a normal playthrough. This does not require 50 completed levels.
In case you missed it: As with many of the other achievements in Rare Replay, this can be easily boosted by exploiting the game save function. Simply save just before a finish line. Load the save, cross the line spinning and get your achievement progress. Repeat until the bloop.
Rewinding will not have the same results. Make sure to snap your achievements!