Models with this rule must test for Treachery at the beginning of each of your turns. Roll a D6. If you roll 2 or more, the unit has passed the test and may fight normally. If the dice roll is 1 then the unit has been affected by Treachery.
To determine what the unit does roll a D6 and consult the Treacherous Gits Table. Note that you do not need to test if the Hobgoblins are already engaged in close combat, fleeing, have pursued a unit of the table or are waiting in Ambush.