Do you have a preference for where you'd like to divert to?
Joking aside, a stack is just another word for a queue. Queing Theory 101 says that in order to sweat the maximum utilisation out of an asset with finite capacity, by far the easiest (though not the only) way is to get the customers to form an orderly queue.
HAL used to publish a factsheet on stacking, but that was withdrawn some time ago, presumably because it was considered a controversial subject: