Survival of the fittest.
Flipper is formed by a diversion of the river around a rock ledge on river left into a notch.
The waterflow is extremely pushy to river right, of which I'll speak more in a moment. The politically correct way to enter flipper is to boof the rock ledge on river left directly into the eddy. This keeps you out of the way of folks who are playing in the main hole. When you finally get to the front of the line, there are three or four fun moves which you can try. Remember that the water pushes to river right, so make sure you're parallel to the current and not to the riverbed.