Which specific feature optimizes the airframe for some species, like the Southern Flying Squirrel?

Answer

Skin folds extending from wrists to ankles

Some species, such as the Southern Flying Squirrel, possess additional skin folds that stretch between their wrists and ankles, further refining their aerodynamic shape.

Which specific feature optimizes the airframe for some species, like the Southern Flying Squirrel?
animalmammalspeedglidesquirrel