This is a doctor-approved and thoroughly tested technique designed to break a baby's cry spell and engage their natural calming reflex. The rhythmic shush reminds baby of being inside mom, where they were inundated 24 hours a day with loud sounds of blood flow and other in utero noises (up to 95 dBs, or the sound of a very loud vacuum cleaner). What may sound harsh at first is actually very calming to a baby.