Both arms have to move forwards or backwards simultaneously (either above or under water). Feet and legs also have to perform the strokes concurrently.
Crawl is the fastest type of movement in the water. Due to the synchronistic, alternating strokes of arms and legs and optimal force is achieved. The face is under water most of the time and is only taken out sideways for breathing.
See “butterfly stroke”
The ball is being moved with the nose or forehead while the player is doing crawl movements.