public void setRenderBackFacesShadows(Boolean renderBackFacesShadows) {
        this.renderBackFacesShadows = renderBackFacesShadows;
        if(renderBackFacesShadows) {
            getPreShadowForcedRenderState().setPolyOffset(5, 3);
            getPreShadowForcedRenderState().setFaceCullMode(RenderState.FaceCullMode.Back);
        }else{
            getPreShadowForcedRenderState().setPolyOffset(0, 0);
            getPreShadowForcedRenderState().setFaceCullMode(RenderState.FaceCullMode.Front);
        }
    }
Set to true if you want back faces shadows on geometries.
Note that back faces shadows will be blended over dark lighten areas and may produce overly dark lighting.

Also note that setting this parameter will override this parameter for ALL materials in the scene.
You can alternatively change this parameter on a single material using {@link Material#setBoolean(String, boolean)}

This also will automatically adjust the faceCullMode and the PolyOffset of the pre shadow pass.
You can modify them by using {@link #getPreShadowForcedRenderState()}

@param renderBackFacesShadows true or false.
jme3-vr/src/main/java/com/jme3/shadow/AbstractShadowRendererVR.java