You should be squeezing/curling both legs. Not just one. Break it down while drilling to make sure you're doing it right. Ask your partner to use 50-70% strength to posture and see if he can regain posture easily at any point. If you are pumping, you should pump, trap the shoulder, deep hook, pivot, c-cup the thigh and finish him any way you like from here. You can chuck him or you can Russian roll or you can try a swim move or you can finish on your back.
The most important thing is that you are squeezing, curling, and generally controlling every single moment of the armbar.