This makes an airfoil surface. Some bats have this membrane covering the legs and tail. The small thumbs are left free for climbing. The membrane gives the wing a stiff edge like an airplane's. Bats that have the tail membrane use it to slow themselves down.
Some bats use the tail as a pouch to catch insects.
Many bats may fold their wings on the upstroke. This helps to reduce air resistance. It is during the downstroke that both thrust and lift are made. The large muscles give powerful wing strokes. This makes flight possible.