From Science:
Birds get all the credit for aerial virtuosity. But bats have some fancy tricks as well, especially at low speeds. Researchers have found a possible reason why: Bats cultivate a unique pattern of turbulence behind their wings. The findings could one day be used to design new flying machines, such as unmanned micro-air vehicles.
At fast speeds, bats and birds fly in much the same way. But at slower speeds, they take different approaches. The reason is anatomical. Birds can separate their feathers on the upstroke to minimize drag and maximize lift. But bats have an elastic membrane for their skin, so they do another trick to keep aloft: They flick their wings backwards and almost upside down.
More here.