Most drawing programs employ the click-and-drag paradigm: left-click on an element and drag it to the destination keeping the mouse button pressed. Although being pretty intuitive, this principle has one disadvantage: it is hard to do something other than dragging, while you keep the mouse button pressed. In particular this means: no zooming (or would you like to press the right mouse button as well, draw the zoom box and then release just the right mouse button ...?). In order to allow zoom and potentially other operations, KLayout employs the pick-and-drop-principle.
In pick-and-drop, you pick an element by clicking at it with the left mouse button, move it (without any mouse button pressed) and drop it (by left-clicking at the target position). Since the mouse button is not pressed, the mouse is free for other operations: just the dragged item is "sticking" to the mouse cursor.
In addition, while dragging the object, Shift and Ctrl keys can be used to force certain direction constraints or override the ones specified in the options (i.e. "move" or "edit" options): The Shift key forces KLayout into orthogonal mode: movements are restricted to horizontal or vertical unless not applicable. The Ctrl key forces KLayout into diagonal mode: movements are restricted to horizontal, vertical or the diagonal axes. Ctrl plus Shift will release all directional constraints - movements will be allowed in any direction.