How do grebes use their lobed toes to swim and dive?

Grebes are specialized diving birds that utilize their lobed toes to navigate the water effectively. While swimming on the surface, they typically use alternate strokes of their feet. When diving underwater, however, they move their feet simultaneously to propel themselves.1

The mechanics of their movement involve rotating the feet 90 degrees between the recovery and power strokes, allowing the broad side of the lobes to be used for propulsion. Just before the recovery stroke begins, the feet are rotated in the opposite direction to minimize resistance.1