The Blunder: You bend your knee while kicking, which causes you to kick from the knee instead of the hip.
Why it’s a blunder: It will cause your legs to sink and bring you out of a hydrodynamic position, effectively creating more drag while not creating any forward thrust.
Your kick actually brings you very little forward thrust, but serves as a stabilizer while you move forward. Especially if you are a triathlete, you want to preserve your legs for the bike and run.
The Fix: Think of your kick as more of a flutter; it’s constantly going like a small motor and helps to keep your body in position. Keep your knees soft and relaxed, your toes pointed, and let your entire leg do the kick, with power coming from the hip. Squeeze your glutes a bit so that you are engaging them and keeping your legs at the surface. The small flutter through your leg should be easy and relaxed.
If you’re able, hold onto the wall with both hands and kick in place, practicing this method. When you’re ready, add kick drills into every swim practice.
The picture below demonstrates the extent to which your knee should bend, which essentially keeps your leg nearly straight. For video and more information on kick technique, check out Speedo’s video here.