What's the Swimboard for?
The board is a buoyancy aid often appreciated by swimmers of all levels. It will help you keep your balance during your series of strokes or undulations with fins. Last but not least, using a board makes the exercise easier.
How can I improve my leg movement?
For effective kicking, the movement should start from the hip, not the knee. The knee will bend slightly and naturally in response to the hip movement, so you don't have to force it any further to bend your knees. Your legs will therefore remain relatively straight. It's important to remember that the upstroke is just as important as the downstroke. Concentrating on the whole stroke optimizes your propulsion and works all your muscles.