On most software when a dialog is on screen you're locked out of the main application until you click OK or close the dialog.
The XPE allows you to have multiple dialogs on screen at once and still interact with the picture. The technical term for this is non-modal dialogs.
For example you can use the 'center button push' feature at any time. (Hold down the center button or scroll wheel of your mouse, and you can push the picture around within the window).
When you have multiple dialogs on screen at once only one is active, and the others take a background role. The active one is shown opaque, inactive dialogs are shown semi-opaque. Just clicking any inactive dialog (or using its key short cut) will make the dialog active.
The active dialog is the one that controls what drags on the picture do. For example if the Rotate dialog is active then dragging on the picture will rotate. If the crop tool is active then a drag will define a crop rectangle.
The standard Windows short-cut of Alt+F4 will close any active dialog.
