The man firing the cannon is touching a glowing match to the touchhole of the the cannon. Smoke can be seen blowing out of the touch-hole as the main charge blows forth from the front of the cannon. At long range cannon balls would be used to take out a file of men. Ideally the balls would bounce in front of the formation and hit the formation with a low trajectory, doing maximum damage to the block. The musket balls would retain a quite tight pattern with devastating effect to the front of an infantry, or cavalry formation.
Some people talk about cannon being loaded with nails and bits of scrap metal.
If this did occur it would only be as a last ditch effort due to the possibility of the metal sticking in the barrel, thus making the piece innoperative for the rest of the battle.