Tani-Otoshi is a great technique to use to throw backwards. Use your hips to feint forwards. Fool your partner into reacting. Use the reaction to throw.
Get uke to react. You want uke to think you are going to throw forwards, so they drop their hips to block you. Do this by twitching your hips, feinting a forward throw like Harai-Goshi.
As uke blocks, slide your leg behind uke’s legs and pull uke down towards the mat.
Direct the throw using your grips on both the lapel and sleeve. Steer uke backwards.