Understanding ALLSELECTED

ALLSELECTED looks like a special function that is able to understand what the user selected in the pivot table, retrieving the original filter context under which the pivot table is running. However, how it is possible that a DAX function knows what a user selected in a pivot table? If the source of a report is not a pivot table but a DAX query, is ALLSELECTED still going to work? This article answer to these questions providing a detailed explanation of ALLSELECTED behavior.