Many factors contribute to the perfect streamline form and mastering this method increases a swimmer's speed.
The body should be on a horizontal plane under the water, with the legs kicking straight from the thighs and hips, not the knees.
Kicking in the streamline position underwater can be substantially faster than swimming any of the other aquatic strokes, competitive or otherwise.
For this reason, competitive swimmers often try to kick in a streamline position off a wall or the starting block for as long as they can be underwater before coming up for their first stroke.
[2] Bad form will cause more drag on a body in water (resistance) resulting in more work needing to be done to cover the same amount of distance.
The other common occurrence of streamline in a competitive race is after a swimmer completes a flip turn and pushes off of the wall.
According to FINA, no swimmer may travel more than 15 meters (16.4 yards) off of a start or turn in the backstroke, butterfly and freestyle underwater.