In reply to Removed UserBoingBoing:
Assuming your taliking about a old fasioned black powder cannon, with a flintlock firing mechanism.
It would have been done from a health and safety point, if the hammer was released prematurely and the cannon were to discharge, the gunner could run forwards along with the path of recoil. ( otherwise he would have to run backwards..)
The raised hand was to indicate that the piece was ready for firing (and to indicate to the horse handler that they were about to fire and to every one else for that matter)Horses were about 40 meters away,
The oficer in charge could see the raised hand and then indicate by lowering of his sabre to the unit comander that he could fire, the unit comander would then give the order to fire.
(ex Dutch Royal Artillery ;which still uses horse drawn artillery for shows)