Hei!
It's so cool that people are making diffusers!
Check out Xenons build thread, where he explains the pictures you linked to:
QRD Prime + Period Question (Live Room)
As for absorption behind panels. A quick way to get a grasp of absorption is to think of the sound waves as water. Whatever material that will get wet when you throw water at it, will absorb. Throwing a cup of water on a diffuser will obviously not make the rearside wet. That's the equivalent to high frequencies hitting the diffuser. Low frequencies, on the other hand, are much larger than any normal size diffuser array. It'll be like dumping a helicopter load of water on the diffuser! The size of the diffuser will be so small compared to the impending wave that the water will wrap around and make the rearside wet too. That is, if the diffuser material is solid enough to block the passage of the low frequency wave. Chances are high that the LF content will partly go straight through the material. In most cases, it'll probably be a combination of diffraction around the object and transmission through the object.
Good luck with your build! Hope you post pictures!
Andreas