It sounds like your turning keeps the pulling of the cable in a direction that keeps your pectoral muscle fully stressed through the pulling, curling of your arm upward and forward. I will have to try the turning.
** I do specific exercises for my job, nurse aide. It requires me to be able to use my one arm to pull things (usually very heavy people!) across me. It's not the perfect situation of a machine. The last time I used a flye machine was last year when I had a gym membership. My total gym enables me to do cable flyes with one arm and I do a little turning, benefits a lot of synergistic muscles too. Usually one day I dedicate one workout to real world weightlifting. One day I'll have to make a video so you and everyone else can see what I do.