A mechanical watch simply takes a reserve of
power, and releases that power into evenly timed

Take a typical water faucet.  The knob can adjust
from full on to barely a dribble.  If you adjust it to
the minimum, you can get the spout to release
individual drips at a regular rate (that regular
drip-drip of a leaky faucet is a good example).

If we take this analogy and translate it to a simple
mechanical watch, the water source (power) is
the wound mainspring, and the balance is the
control valve.  The balance is the wheel  that you
see rapidly oscillating when you open the back of
a watch, incrementally releasing the power from
the mainspring. From there a series of gears
(power train) in between the mainspring and the
balance step down the speed, and run the hour,
minute, and second hand.

A (somewhat) simplified explanation of how a mechanical watch works:
For a good movie, created by the
Hamilton Co. in 1949, see
How a watch works
on the Internet Archive project.

Another great video can be found
here on

About gear train of wristwatch.