The modern breaststroke kick: heels to the seat, feet flexed and turned out, then whipped back and together. The knees open no wider than your hips.
In breaststroke the larger share of propulsion comes from the legs — and that same kick is your biggest brake as soon as you make it wide. What decides it isn't the knees, it's the feet: only a flexed, outward-turned foot grips water, while a pointed foot slips straight through. The classic fault is the wedge or frog kick: knees splaying wider than the hips, legs pressed apart with pointed toes — little drive, plenty of drag, and strain on the inside of the knee. Rebuilding it feels wrong at first, because the old pattern is grooved in: heels close to the seat, knees no wider than the hips, toes pulled to the shin, then whipped round and back together. What often gets missed: a scissor kick, where only one leg whips cleanly, costs more than speed — it twists one knee asymmetrically. You will barely feel it; on video from behind you see it immediately.