You probably came across this video of a pediatrician calming crying babies in a magical way. It does seem amazing, the baby is calmed so quickly it almost seems like some sort of hypnoses.
But in realty there is no magic here. The doctor is only taping into one of the baby's most basic needs- movement.
In the uterus the baby was gently rocked when her mother moved. The movement helped her calm down and fall asleep. That's why at night when the mother slept the baby moved around more, there were no movements to gently rock her to sleep.
Now that she's born the baby still needs those calming movements, and if their reminiscent of moving under water and done while she is held tightly like she was by the uterus walls, all the better.
You can get the same affect by holding your baby and gently bouncing on an exercise ball or holding her close to your chest and gently rocking.