"Yes, it's still Ambient Occlusion, but it renders MUCH faster than AO on the light would do." It renders faster, most likely, because you aren't raytracing AO for every single object in the scene, like you are when you use it on a diffuse IBL. OcclusionMaster allows you to put AO via a node on certain mats in the scene, while not having it on others. You can do this without OcclusionMaster too, but Face_Off's script makes it MUCH easier to control. It's the same AO as is on a light source, only applied on materials via a node instead. It's still raytraced, etc.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.