Originally Posted by IngeA
No, the legs are allowed to be apart. But when they are closed during the kick like in a breast stroke kick, it's forbidden.
As the camera is not exactly behind the swimmer, it's not to be seen properly, but for me it looks like there ist the snapping, closing motion (whip kick) like in a breast kick.
The definition of a butterfly (dolphin) kick in USA Swimming rules is:
"All up and down movements of the legs and feet must be simultaneous. The position of the legs or the feet need not be on the same level, but they shall not alternate in relation to each other. A scissors or breaststroke kicking movement is not permitted."
There is nothing about whether the legs are together or apart.
The rules for breaststroke kicking say:
"After the start and each turn, at any time prior to the first breaststroke kick a single butterfly kick is permitted. Following which, all movements of the legs shall be simultaneous and in the same horizontal plane without alternating movement.
The feet must be turned outwards during the propulsive part of the kick. Scissors, alternating movements or downward butterfly kicks are not permitted except as provided herein. Breaking the surface of the water with the feet is allowed unless followed by a downward butterfly kick."