Note the man (second from the right) with his hand on the cannon. He is the number 3 cannoneer, and he is covering the vent to keep air from any embers that might be present in the bore.
Cannon Firing
Demonstration
As the number 2 cannoneer rams the charge into the muzzle, he quickly shoves the ramrod in with his hand facing palm-up. My guess is that there have been instances where there were embers in the bore that have ignited the charge, and this is a safety measure to protect his hand.