"Butterfrog" stroke is a variation of butterfly allowed in US Masters swimming, but not permitted under FINA swim rules. In butterfly, two dolphin kicks are permitted per arm stroke. In butterfrog, the breaststroke whip kick is used either one or two times per arm stroke.
FINA SW 8.3 All up and down movements of the legs must be simultaneous. The legs or the feet need not be on the same level, but they shall not alternate in relation to each other. A breaststroke kicking movement is not permitted.
US Masters 101.3.3 Kick—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. The breaststroke kick may be used exclusively or interchangeably with the dolphin kick while doing the butterfly stroke at any time during the race. Only one breaststroke kick is permitted per arm pull, except that a single breaststroke kick is permitted prior to the turn and the finish without an arm pull. After the start and after each turn, a single breaststroke kick is permitted prior to the first arm pull. A scissors kicking movement is not permitted.