The Herders may light the Bulls at the start of the Movement phase. When the Bulls are released, you may choose the direction in which they travel, and the point on the concealing unit from which they start. Fire Bulls move 3D6" using the rules for Random Movement.
After release, the Fire Bulls have Immunity (Psychology) and are treated as individual units from their Herders. In your subsequent turns they move in the Compulsory Moves sub-phase using Random Movement, but they always move in a random direction.
If the Fire Bulls' move would bring them into contact with another unit, then they move through rather than stopping. If the Fire Bulls' move would end within a unit, then they automatically move through it — place the Fire Bulls 1" beyond the unit, in the direction they were moving. When a unit of Fire Bulls moves through a unit (friend or foe), each Bull inflicts a S 5 hit with Flaming Attacks.
Any unit that moves into contact with the front arc of a unit of Fire Bulls suffers a S 5 hit for each Fire Bull in base contact. These are resolved like Impact Hits in the close combat phase if the Fire Bulls are charged, or immediately in case a unit flees through them.
When suffering casualties from shooting or magic, the Herders follow the rules for allocating hits on unit Champions. If the Herders are killed before they have released the bulls, the bulls are removed as casualties. Herders never count for the purposes of Victory Points.