It is primarily used to determine the appearance of off-table troops such as reinforcements or flank marches, but can easily be applied to other events as required. Although dubbed the 'Alan Roll' by other members of the Staines Wargamers, I don't claim to have originated the technique. There is no randomness in this method and allows advanced planning based on their appearance. Reinforcements appear on Turn x, where 'x' is a random number. This fixes the turn the reinforcements appear, so the player can plan based on this knowledge.
Reinforcements appear on a given roll.
This is the method used by DBM to determine the appearance of flank marches. They appear on the turn after the owning player rolls a '6' on a D6. This method makes their appearance unpredictable, but the time frame in which the reinforcements appear is very variable, perhaps making them too unpredictable. It is possible that the reinforcements will never appear. Reinforcements appear according to an increasingly easier random roll.