The momentum of pushing water backwards propels swimmers forward in the water, thereby creating kinetic energy in the water that goes backwards and produces forward movement. A swimmer's hands and feet provide lift and backward momentum in the water. Swimmers' hands act similarly to an airplane wing moving through air, except hands are like hydrofoils moving through water.
A hand is similar to a curved foil that produces different water pressures on either side. When the hand is curved in the water, the back of the hand acts like the top of an airfoil. Fluid travels faster around the curved part of the hand rather than the palm. This force makes swimmers more efficient moving through the water. Swimmers maximize this energy by sculling their hands in an S-motion through the water. The S-motion leaves the hand in the water for a longer period to further increase momentum.
Kicking also propels swimmers forward. Kicking motions do not produce as much lift as hands, but keeping legs as close the body's centerline as possible reduces drag in the water. When legs form a straight line with the torso, head and shoulders, there is less surface area in the water to increase drag. Drag is caused by friction and naturally slows down objects moving through air and water.