The rooster tail is a hydrodynamic effect. Dall's porpoise breaks the surface at a shallow angle while travelling quickly, and the head pushes a cone of water upward and backward as it clears. The animal breathes inside that hollow, so it takes a breath without properly leaving the water. Speeds of around 55 kilometres per hour have been reported in short bursts.
The body is built for it: unusually thick and muscular for a porpoise, with a small triangular dorsal fin and small flippers set well forward. The black-and-white pattern, with a large white flank patch, is often mistaken at a distance for a young orca.
Dall's porpoises frequently bow-ride, which is unusual among porpoises, and their willingness to approach vessels makes the rooster tail the most reliable field identification for the species. They rarely breach clear of the water in the way dolphins do.