shadows only -lights

sometimes they use a trick to cast only a shadow. no lighting. so they position a light and make it cast only a shadow. they dont use 100% black shadows. sometiems you want to add a shadow somewhere but without the lighting  for a dramatic effect.
poser i think doesnt have an option to make only shadows.  but we can use negative lights.

so what we do is:
-we position a light that we like.
.we duplicate the light
-the first light is a positive number. the second light is the same number but negative. the second light doesnt have shadows.  the second light will take away the lighting but leave the shadow.