You must polarize your light source and polarize the light coming into your lens such that your camera cannot see your lamps. This way, light shining on your subject and not reflecting directly off of a shiny surface is seen because its polarization pattern is randomized by the irregular surface reflections of a nonspecular surface, whereas specular surfaces will not be seen as much because they will preserve the polarization.
This is a technique used to shoot a lot of shiny objects such as pottery items. It's very important to show their patterning without so much surface reflection, and polarized light helps with that.
You will need to set the polarizer on the flash so that it is black when viewed through the lens of your camera. If you have an SLR, this is relatively easy, but it's difficult with a rangefinder camera. Remember that you're going to lose a couple of stops of light because you're excluding so much. If your flash's polarizing filter appears black to the lens of the camera, reflections are minimized. If it appears clear, reflections are maximized. What you're doing is then illuminating the subject with vertically polarized light and photographing the horizontally polarized light coming off the subject. It works very well.