I think it is a reference to the iPad "zoom" feature.

If you double tap here: x

Then the page zooms in to this spot, on the iPad turned sideways, it should zoom into the text of the message. You can't see the ads on the right, but you can't see who the poster is either! A double tap on the same x takes it back again.

Double tapping on the left of the screen (on the white area below your user name) zooms into that point and cuts off the right of the screen.

Basically the iPad focuses on the point where you double tap.