If focus is on an unselected item, pressing Shift-Down selects all items between anchor and the item which is right below the focused item. The item which is right below the focused item is also included in the selection. All previous selections are canceled if aplicable. Available in multi-selection mode only.

For more information on what is meant by anchor on Windows, please see this article:
http://msdn.microsoft.com/en-us/library/ms971323.aspx#atg_keyboardshortcuts_anchoring_and_exte