Use the transitions preferences pane to enable changing spaces by moving your mouse to the edge of the screen.
If you choose to use this feature without any modifier key selected, the system will activate the desktop change after your mouse cursor has been at the edge of the screen for about 1 second.
If you select one of the modifier keys (shift, ctrl, alt or cmd), then you must hold this key down and move the mouse to the edge of the screen in the direction you want to go for the space change the happen. In this case the system will activate faster as there is less chance that space changes will be activated by accident.
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