Within breaststroke there is room for variation and the leg action can either be a narrow “whip kick” or a wide “wedge kick”.
A whip kick is often used for racing swimmers. Keeping the thighs narrow makes the kick more streamlined and therefore the swimmer travels more efficiently through the water. The lower leg rotates around the knee joint, which for some swimmers can be uncomfortable.
A wedge kick is a wider kick where the knees draw out to the sides, the feet then push out wide, before the legs snap closed. Because the knee comes out wider than in a whip kick, the knee joint has less rotational stress, which can be more comfortable for some swimmers. Many children or beginner adults learn a wedge kick because the pattern is clear and easy to understand. It can be modified or tweaked later to become more of a whip kick if the swimmer prefers.
Here is a video demonstrating the two types of kick: