Ushiro nage = ushiro ate in Tomiki term. Click on link
to see animation of ushiro nage/ate. The picture show kata version, but many nasty thing can come out of this technique if you follow through with it e.g., hadaka jime, okuri eri jime or kataha jime (all judo choking techs).
I am no good with description of techniques using words, I hope the picture can help me describe it better.