The advice is to make your patty with an indentation in the center (see photo). That way when the center of the beef patty swells or plumps when you grill it, the burger will revert back to a normal / flatter patty shape that will nicely accommodate a soft bun. Simple enough.
We tried the technique in the BBQ test kitchen and must say were surprised at how well the “indentations” on the test patties disappeared. We probably could have made the dent evendeeper (or on both sides). The control patty (“test” or “not concave” is on the left in the photos) indeed DID expand in the middle… But honestly not enough to bother me.
Another recommendation would be to control yourself and keep from pressing the burger down on the grill with the spatula. I don’t know why guys do this but I see it done all the time.
All-in-all the tip is worth while and is good general practice for the next time you’re pressing patties.