Swimming kick explained
The kick is not the same in every stroke
Freestyle and backstroke usually use an alternating flutter kick, butterfly uses a dolphin kick and breaststroke has a distinct simultaneous kick pattern governed by its own rules.
What the kick contributes
The legs can contribute propulsion, body position, rhythm and stability. How much propulsion they provide varies by stroke, event distance and swimmer.
Sprint versus distance
Sprinters often use a more forceful, frequent kick because speed is the priority, while distance swimmers may use the legs more economically to support body line and timing.
Legality matters
Breaststroke and butterfly kicks are much more tightly defined by rule than freestyle. A technically effective movement that violates the stroke rules still makes the race illegal.
Internal links
Link to stroke guides, sprint and distance swimming, underwater dolphin kick, breaststroke rules and stroke efficiency.