I have tried both ways myself, when i was a 2 plane swing I would use my hips to start my downswing, and when i was 1 plane I would use my left arm to start my swing.

the last 6 weeks I have just focused on swinging up slowly with everything together and then down slowly with everything together, then hitting hard at the impact zone. I don't know if I am 1 plane or 2 plane because have not looked or put it on cam, but I know I am on plane through impact and hitting a small draw, with plenty of backspin on my short irons.