just got baffled with this question myself after watching how nimbly Chris Reece switched between zoom in and out
as David said above, in the Key Commands find zoom related commands, assign keys to 'Save As Zoom 1' , 'Save As Zoom 2', 'Recall Zoom 1' and 'Recall Zoom 2' options
in Arrange create zoom out of the entire arrangement, save as zoom 1, then zoom in on a particular region till you like the level of magnification and save as zoom 2, then it's a matter of switching keys on the keyboard to toggle between the two having a particular region highlighted