That's the final stage before swimmers break the water surface. Here, depending on the stroke, athlete initiates movements with their legs and arms.
Freestyle, backstroke, and butterfly all have the same underwater phase: dolphin kick in a streamlined position. The breaststroke underwater phase, a pullout, consists of a powerful pull down with arms and one breaststroke kick.
The key here is to transfer all the speed and power into the first stroke that will set you up for a good race.