This theorem is a consequence of the principle of conservation of energy, applied to ideal liquids in motion. As per the theorem statement that is for the streamline flow of an ideal liquid, the total energy (sum of pressure energy, potential energy and kinetic energy) per unit mass remains constant at every cross-section, throughout the flow. In the 18th century Daniel Bernoulli noticed that fluids flow faster when forced through constrictions. We usually come across these kinds of phenomenon when we observe and compare rapids and meandering rivers.
Bernoulli reasoned that when a particular energy increases (kinetic) in a system the rest of the forms of energy must decrease .
To understand this principle the best example would be observing the lifting of an airplane. An airplane wing is shaped in such a way that the air passing over it must travel further and faster than the air passing underneath. This leads to a lower air pressure build up on the wing than the air below. This produces the lift force which keeps the airplane afloat.
You have probably experienced driving on a highway and having a large truck pass you at high speed. In this situation, you may have had the frightening feeling that your car was being pulled in towards the truck as it passed. We will investigate the origin of this effect in this section.