The third section in the Triggers tab is the Drag & Drop palette. Opus can automatically have an action (or a series of actions) happen when objects are either dragged or dropped.
Note:
These triggers will only affect objects that have been set up as drag objects or drop zone objects. To set up drag and drop objects, see the Introducing the Drag and Drop feature and the Drag and Drop tab for more information.
You will need at least one object that can be dragged and dropped and at least one object that is set to receive dropping objects on the current page. There is nothing to prevent objects having both options set so the object can both receive dropped objects and also can be dragged and dropped themselves.