Room treatment - diffusion question

I have seen many picture and have read lots of articles on where to place difusers. One of the places suggested is in between the main speakers to increase the soundstand and improve imagine. From what I have read, diffuser mainly work on the mid and high frequences and this frequences, unlike bass frequencies, are directional. This being the case, how does a difuser, placed between the mains help when the sound it is difusing is directional? Is it difusing sound coming back? I can see how a difuser might work on the side walls, but on the wall behind the speaker, I am not sure.

You might be surprised how much "coherent" energy is radiated behind speakers. It is then reflected and out of phase with the direct sound from the speakers. Diffusors behind the speakers break up these waves and make them less coherent. You still get some radiated energy, but it's scattered, thus minimizing the "in phase out of phase" response at various frequencies at the listening position. In general the result is a cleaner sounding system because you are hearing relatively more sound of the direct component vs the radiated.
Above 300 Hz, there's an incredible amount of "bouncing around", and Rives is certainly right about even speakers with no obvious "back wave". Read F. Alton Everest's "Sound Studio Construction on a Budget", and you'll see that he favors two-dimensional diffusors, RPS Skylines, in fact, on front and back walls, in various kinds of listening rooms. My experience is that they do a freat deal of good there. I think side reflections are the trickiest: you need them but not too much strength in them, it can be hard to get right. But diffusors on front and back walls is a no brainer, IMHO.