When you drop on a target object and the source object has said it knows how to handle the drop operation, this method will be invoked on the source object. For example, the program object class supports being dropped on certain other classes of object where the drop action will be taken by the program itself. The drop action provided in WPProgram's override of the wpDroppedOnObject method would be to execute itself as a viewer of the target object. This method is called as a result of a DM_DROP message being sent.