Initiates when a form filler performs an action that causes the drop-down list to appear, such as clicking the arrow icon on the drop-down list or by tabbing into the drop-down list and using the down arrow. This event initiates before the contents of the drop-down list are displayed.
You can use this event to control the loading of large numbers of list items. For example, you can use this event to load a fixed number of records from a data source into a drop-down list. This improves the performance of the form for the form filler at run time.
See also 

preOpen event