The illusion of soundstage.

If you put your drivers out of phase with each other you will get sound all over the place. Q-Sound is software that manipulates phase to put sound beyond your speakers , even behind you
Alan

Or, in my case and situation, I'm running Walsh drivers which are omnis.  I've a 'soundstage' that 'extends' not only to the sides but also in front and behind the drivers.  Placement within the given space and the surrounding objects (or lack of same) will certainly effect 'the effect'.  I've also noted that from selection to selection, artist to artist, album to album there's distinct differences in what is perceived where.
Adding a second pair, either in 2 channel or 4 channel arrangements, and playing with delay or no, you can be sitting 'on stage' or row 5....
But 'immersion' isn't to everyone's taste.  To each...

I think a wider soundstage is a result of proper setup for I have yet to hear a pair of speakers that do not pass beyond the drivers. Ex: with Dyn's I found very little to no toe in extends the soundstage way beyond the speakers and still retain great imaging but somewhat close to each other. Toe them in and the imaging gets a hair better but looses the wide soundstage. Raidho/Scansonic on the other hand require a farther distance apart but with a fairly severe toe in. No toe in creates an even wider soundstage but then severely looses the imaging.

Pan pots used to any degree by any recording engineer are the secret…pan left, pan right, or no pan at all.

Head Related Transfer Function. :) The secret is how your head, ears, and hair affect incoming sounds from different angles. It’s more than just amplitude and simple phase. Your body adds comb filtering your brain picks up on to determine vertical and horizontal angles. Very cool science.

Having said that, to get good sound staging in any dimension you must have good room treatment in that direction. So, if you want good left to right soundstage, you should have good room treatment to the sides and behind the speakers. If you want height, good treatment above the speakers.

There’s a known speaker gimmick. Dipping the frequency response around 2.4 kHz will exaggerate the sense of imaging. You’ll see this in a number of "high end" speakers.

Most of the times I've heard this effect by the way it seemed like an accident, as opposed to a real attempt by the recording engineer to do something. Intimate jazz quartets for instance.