Some bike helmets have a smooth curve in the back and many others have an extra bump.  The one I wear, Specialized Align II MIPS does.   Why the bustle? A vented helmet creates turbulence so a rear bump will have little effect.   I think shaving my goatee would have a greater aero effect than a helmet bustle.  I think the main purpose is styling or “aero look”.  

Those with a bustle

Smooth Rear Curve

   A week ago there was a record-high temperature.  Lots of news people gave all sorts of warnings about staying inside.  Obviously, I went for a bike ride. I went south on the LFT , Lake Front Trail.  When I got to the South Side Cultural Center I rode to the beach in the back.  I decided to take a swim, even though it was before the lifeguards.  I removed my shoes and waded out. Then I discovered a great advantage of the helmet bustle.  When you float on your back the extra styrofoam is buoyant enough to keep your head at a comfortable angle in the water.  Very relaxing, you could almost go to sleep in that position; until I noticed the west wind was blowing me farther out into the lake.