In all modes, when the cursor is over the active photograph window, you can press and hold the ALT key to bring up the temporary Zoom Window.
The temporary Zoom Window zooms the image under the cursor to a fixed magnification and specified size. These are set in the Preferences dialog. The current zoom level can be changed using the mouse wheel.
You can carry out most actions in the Zoom Window, such as selection, moving by dragging, marking points, marking lines, marking curves, etc.
When you are finished the desired action release the ALT key and the Zoom Window will go away.
When the ALT is held down and the cursor moves outside of the Zoom Window, the contents of the window will pan in the direction of the cursor. The speed of the panning is controlled by settings in the Preferences dialog.
Note that the Zoom Window displays all those model items that the user has specified in the Visibility On Photos pane.
If you find the Zoom Window is too slow to appear you may try 1) reducing its size, as specified in Preferences, or 2) changing model visibility for those types of objects that you do not need visible for the current work.
The Zoom Window is especially useful when you are dealing with large images (for the whole image to be visible you need to use a zoom factor such as 1:4). If the Zoom Window magnification is set to 200% (2:1) in Preferences then you can easily get an overall view of the photograph while staying at 1:4 and doing detailed work (marking, moving, selection etc.) by using the Zoom Window at 2:1.