The fully extended body position after starts and turns — arms overhead, hands stacked, head between the upper arms.
Water is roughly 800 times denser than air, which makes drag by far the biggest brake in swimming — more important than strength. The streamline exploits that: one hand over the other, thumb hooked, arms pressed against your ears, core braced, legs together. In that position you glide considerably further off a push-off or turn than in any other. The most common fault is a head that looks up between the arms or hangs down — either breaks the line and acts like a parachute. Second fault: starting the arm pull too early. Glide until your speed drops to swimming pace, then begin.