Winds blowing from different directions at different altitudes and speeds, cause the air around it to begin rotating horizontally.
When this horizontal column of air meets one of the strong updrafts associated with a thunderstorm, it can become twisted and bent upward. This mass of rotating air is known as the mesocyclone. It starts at ground level.
Once the storm begins to rotate the energy level begins to increase. According to the Bernoulli Principal of physics (discovered in the 1700's and accepted as experimental fact ever since) it must remain constant. The only remaining potential to keep the energy level constant is for the pressure within the system to drop. It is the same principal that proves itself every day in the lifting of an airplane and the throw of a curve ball.
As is the case with any other system the storm will take the path of least resistance. The lower pressure has least resistance upwards where the pressure is already lower. So the storm climbs toward the lower pressure of the upper atmosphere.
As the height of the storm increases, the pressure of the mesocyclone at the ground decreases which increases the rotation of the near ground level rotation which further increases the wind shear speed which further decreases the supercell's pressure etc. etc.etc...
Warm air has met cooler air. We know that warm air rises. When it does it reaches cooler air and condenses to form clouds. When that happens quickly it is an updraft. The updraft seeks the path of least resistance and grows higher and higher.
A decrease in barometric pressure centers upon a point at or near ground level that is the new path of least resistance for the dropped wall cloud. The wall cloud seeks to meet it. A funnel shaped structure of cloud base is pulled to that new lower pressure point. This is the tornado's birth.
As the new tornado reaches down to its invitation it follows the point of lower pressure. That point will move causing the tornado to swing from one side to another in what appears to be no relative sense to the storm system above it. It is the connection point between the two halves of the supercell storm system.
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