What does Napoleon do to chase Snowball off the farm?

Napoleon does not really do all that much to chase
Snowball off the farm.  Instead, he relies on his dogs to do it for
him.

At one point, Snowball is speaking at a meeting. 
Napoleon decides it is time to get rid of him and he emits a high pitched sound.  That,
apparently, is the signal for the nine dogs to chase Snowball.  Snowball runs for his
life and leaves the farm.

So all Napoleon really does right
then is to emit the high pitched sound.  Other than that, it's just that he had trained
the dogs.