HS Back Roll spin help

So I went through the whole phase about which invert to try first out of the hs back roll and a tantrum. I ended up trying and landing the tantrum first because I used to back flips all the time as a kid and it easy for me to picture the spin in my mind. Now that I can grasp that trick alright, I wanted to give hs back rolls a shot but have no clue how to spin them or even start the rotation. Most of the tutorials say keep the handle close to the body, stand tall of the wake and look over the lead shoulder. It's the looking over the lead shoulder part I don't get. Maybe because when I do a back flip, I know I have to snap my head back to initiate rotation but in the case of the back roll, I don't really understand which part of my body is supposed to do what. Any help in detail would be awesome. Thanks!