"Whiffling" is a term used in ornithology to describe the behaviour of geese (and some other species) where a bird rapidly descends with a zig-zagging, side-slipping motion. It is achieved by temporarily flying upside down. This is how it is done.

A bird’s wing feathers are mounted like slats on a venetian blind. They only lock together to form a solid aerofoil against airflow from below. If they fly upside down, the feathers rotate open and let the air through which modifies the flight dynamics. Modern aircraft are designed to get the same effect (without inverting the plane!) by opening slats in the wings: this is done while landing or performing manoeuvres that would take the aircraft close to stalling speed.

But how could a bird control its flight in this position? When geese use the whiffling manoeuvre, at the same time as they roll their body upside down they also twist their neck through 180 degrees so as to keep their head in a normal position, the right way up. Full visual control is maintained.

The overall effect of "whiffling" is simple and dramatic. The aerodynamics (that usually give lift while flying) are inverted and the bird briefly plummets toward the ground before this is reversed and it adopts a normal flying orientation. This erratic motion resembles a falling leaf. The manoeuvre lets the bird rapidly lose speed and height for a fast landing. It is used to evade predators or to avoid a slow descent over an area where wildfowling is practised.

More Info: en.wikipedia.org