Power Select can be cycled to one of three settings for all Blender modes; Tweak, Drag, and Smart. By default, box selection will always automatically occur when dragging the mouse not directly over an object while in all of these modes. The different functionality occurs when clicking and dragging on objects or edit mode items.
Tweak: Immediately moves all selected objects/items when clicked and dragged over.
Drag: Drag/circle select objects/items when clicked and dragged over. Hold Shift to add to selection. Holding shift and dragging over already selected items will deselect those times unless “Shift Drag Deselect” is unchecked in the Power Select options menu in which case hold Ctrl to deselect when dragging..
Smart: Drag/circle select objects/items if not previously selected, if already selected then tweak. This option is especially useful with a tablet.