A stab to either kidney is a very viable option.
"A stab to either kidney is a very viable option. Puncturing a kidney with a knife and leaving it in can create an anchor point on the opponents body, literally its like creating a handle. I can help you move him where you want. The Kidneys are attached by Suprarenal veins, so even if the kidney is not punctured, a thrust here may still sever one of these and result in rapid loss of consciousness and a lot of pain.
A stab to the kidney will make the opponent stretch out and instinctively try and turn away from the origin of the pain. Use this..." Notebook