Kicking flat is like standing straight up and down with both feet on the ground. Kicking on your side is like standing on one leg (which gets hard and annoying after a while). When you kick on your side ensure that your head looks DOWN at the bottom. Only turn your head to the side to breathe. After you take a breath, snap your head back into position looking at the bottom. Because the whole body is rotated about 45 degrees up onto its side, your kick can’t do as much to keep your legs up. You will achieve the best body position by engaging your core, extending your one leading arm high in the water, and doing your best to push your chest down in the water. Kicking on the side is particularly tiring to the legs.