Quoth Sheldon "Better Than Round" Brown:

Biopace is a patented non-round chainwheel design made and licensed by Shimano. To a casual glance they resemble elliptical chainwheels, but on closer examination they turn out to be the exact opposite of the classical elliptical design. The product of extensive research and computer-aided design, Biopace chainwheels have the small radius engaged when the cranks are horizontal, the large when they are vertical. This is because the Biopace design is based on a dynamic analysis of the motion and momentum of moving cranks and legs, unlike the static, geometric analysis that produced classical ellipticals.

Biopace chainwheels were made by Shimano and were also made under license by some other manufacturers. The shape of genuine Biopace chainwheels is not a simple ellipse, but a more complicated shape which Shimano describes as a"point-symmetric egg curve." Other crankset makers have made similar designs, generally of simpler shape (pure elliptical) but oriented in a similar manner to Biopace chainwheels. Examples are SR's "Ovaltech" and Sugino's "Cycloid."