I can't twist my leg like that unless I'm seated lower and legs are almost straight. Made a mistake of working out thigh muscles and all the last time so I'm not exactly flexible. What you're saying is slide from back pedal to throttle right? I can do that with certain shoes. Unfortunately I'm not either big nor small nor flexible. Bloody inbetween.
I guess it is more of the matter of having understanding how the car react, and what you can do with your body, i dont think anyone born with a natural reflex to do heel-toe.
I think i can say that i can heel toe in almost any car/shoe/slippers once i get comfortable with the position, of course have to do some homework, such as
-figuring how the shifter react, the position
-pedal position, leg position, depressed brake height is it comfortable to heel toe - to get along with awkward position (especially in a classic mini-story below)
-hand and leg coordination
-how fast the rev builds
-gear ratio
the worst one will be classic mini with stock seat, odd angle steering, non-centering type - 4 speed manual shifter, odd pedal position and angle, steering shaft is between brake and clutch and it is very close to the brake pedal doesnt help as well. Besides that, the fact that the brake and gas pedal being really small and close together doesnt help either(sometimes i cant brake during earlier period of practice because i cant let go of the gas fully), and not to forget the "big" cabin space. took me few attempts to be able to heel toe it. Also, your knee and the steering is already so close together, it just make things worst.
i think the best way to start is to learn how to control the braking force with free hanging leg, do not pivot/support with your ankle on the ground. I mean the ankle do not touch the ground when you're using your brake - i am not good with english as you can see all the shit grammar
After that you can learn how to rotate/twist your leg, be it heel-toe or toe-heel, you choose your own flavour. happy practicing guys