At the begining of the fire phase, instead of firing, A unit armed with at least 1 weapon of Heavy 3 or more can elect to place an ordinance template on the board during its fire phase. If the center of the template is out of range it is removed with no further effect.
If, in the enemy movement phase, any models attempt to pass through that zone they are hit once each. If the firer attempts to place the template over enemy models the fire is resolved as normal instead. If a squad contains more than one such weapon, then enemy models will be hit once for each weapon in the squad that meets the criteria Heavy 3 or greater. Regardless of other factors, a single unit may only ever place one beaten zone template a turn.