What Is The Bernoulli's Theorem?

Bernoulli's theorem states that:

When a fluid flows from one place to another without friction, its total energy (kinetic + potential + pressure) remains constant.

An important corollary of this theorem is:

Pressure in a fluid decreases with increased velocity of the fluid.

When the piston of a sprayer is pushed, air is forced past the upper end of a tube, whose lower end dips in the liquid to be sprayed. Due to the increased velocity of air, the pressure near the upper end of the tube is reduced. The atmospheric pressure in the container, therefore, pushes the liquid to the top, from where it is carried away by the stream of air.
When the bowler spins a ball, it changes its direction (swings) in the air due to unequal pressure acting on it. Due to spin, the wind velocity is increased above the ball and decreased below it. This creates lower pressure above the ball which, therefore, is lifted upward.

The shape of the wings of an aeroplane is such that wind velocity above the wings is higher than that below them. Consequently, there is higher pressure below the wings and this lifts the aeroplane.