Flu-flu arrows have broad feathers intended to slow the arrow after a short flight, usually around 40 yards. The feathers stand high off the shaft and flatten just after the arrow is loosed and it has its highest velocity. After dropping below a certain velocity threshold the feathers stand back up and provide a sudden increase in drag, bringing the arrow to a halt. The idea is that you can shoot into trees at squirrels or birds and your expensive (or labor-intensive) arrow won’t fly into the unfathomable depths of forest. People also often tip these arrows with blunt heads to keep them from burrowing into the ground or penetrating deep into impromptu targets when roving.
But why the funny name?