Shadow Only Plane In YafRay

Well I’m testing and testing and i’m loving Blender more on each minute, but now I wanna try to get a backplate to light up using and HDRI image to render, but I got no shadows.

I added a plane under the objects and ticked the OnlyShadows on the Shaders, set up a Sun too but still no shadows. I’m Using YafRay WIth Direct Lightning and IBL turned on.

What am I missing here?