Incendiary pigs are 'one shot' weapons intended to spread panic and terror amongst enemies, particularly mounted troops.
The pigs are coated in pitch, tar and oil, and herded towards the enemy. At the right moment, the pigs are ignited by their handlers and, not unnaturally, they run away in pain and terror - hopefully towards the enemy. Apart from goring anyone foolish enough to get in their way, the pigs are tremendously disruptive to formations. They are also very frightening for elephants in particular, and this is their main use in warfare.
Pigs can only be fired up once during a battle, and few survive for long.