The Leg Movement
The legs are moved up and down in a continuous action, and not necessarily in co-ordination with the arms, the motions being free from the hips and not from the knees, keeping the legs straight, without having them bent much at the knees (a fault almost all beginners have) and pointing the toes out straight.
However, after the swimmer has mastered the leg action, he can bend slightly at the knees, in order to get the sort of propelling movements necessary to help move the body in its steady pace through the water. But remember, the legs do not play so large a part in the crawl stroke as the arm movements, and are used mostly to keep them afloat and accelerate the run, and the fellow who gets it into his head that the more work and greater fuss he makes with his legs the faster he will go, will soon find that he is retarding his movements and not getting the full speed.
While there are definite and fixed movements for the crawl, hardly two swimmers use the stroke exactly alike, but all of them must nearly approach the proper position in the water, get good balance, breathe regularly and with precision on each stroke, and relax more in the crawl than any of the other strokes, if a clean, easy movement through the water is desired.