This isn’t in their documentation. I wrote to the help desk and asked:

When the user stops scrolling, they can see everything in their browser viewport. So how is the impression line calculated? Is it…

a) the “cut off” line at the bottom of their viewport
b) the “cut off” at the top of the their viewport?
c) something else?

Response from Crazy Egg helpdesk:

Everything within the viewport will receive an ‘impression’ count. So as you navigate a page thousands of impressions are added up based on the viewable portion of the browser. The line in our report gives you the exact number of impressions for each specific part of the page

So basically, it’s what you would hope for!