The flexpole doesn't always have tension on the dog. I see its utility being that if the dog begins to lose focus or drive or do anything during heeling you don't want him to do, you can just step out/away from the dog, which will build frustration and drive/focus and then step back into a correct heeling position and reward. With this approach, you don't have to expend your or your dog's energy teasing him up with a toy and misses and then going into heeling. This approach saves energy for the actually heeling. Flexpoles are not cheap at around $300 for an adult dog. That is why some use a long line on a tree to reproduce the effects.
I'll ask again, where do you hold the food when training heeling?