In most cases sprinters get deep to go fast. They do so because...

They do so because they have to create as much propulsion as possible in as little time as possible.
They create a lot of propulsion by using a big surface area to move a lot of water backward over a large range of motion.
You can see how long he keeps everything facing backward.
This is the key. All the speed is in the back half of the stroke.
More propulsion in less time means more speed.