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If you thought that achieving this feat has anything to do with involuntary and jerky movements induced by the fish's nervous system in the muscles, you were wrong. The answer to this phenomenon lies in fluid dynamics, the body of many fish enjoys a "topography" that manages to direct the energy of the vortices that naturally exist in turbulent waters in propulsion.
Fish may not know anything about fluid mechanics, but billions of years of evolution has led to the development of this very refined technology. This phenomenon was discovered by scientists at MIT and Harvard who were studying exactly whether it would be possible to "collect" part of the energy present in the vortexes of turbulent water flows and use it for propulsion. That's when they discovered that nature had already solved it.
Because of this phenomenon, we found that many fish expend little energy when they need to swim upstream. Under the right conditions, they can literally "let go of the body" which will naturally be propelled.