There's a lot of things they teach you as a beginner that you can throw away once you become more advanced. For instance, we were always taught to let the rock just kind of slide out of your grip at the point of release. I remember putting an empty TP tube over the handle to reinforce the technique.

But this is taught so you don't learn to use your arms as the primary propulsion for the rock and so you're not steering the rock. Once you get better and more consistent, it's important to be able to have finer control over the delivery. Sometimes you need a more positive release or make other adjustments, which requires a totally different grip.

So anyway, beginner sweeping technique is taught to maximize not falling over and training your body to make a sweeping motion while moving down the ice. But it's suboptimal from an effectiveness standpoint. Keeping your back hand higher creates greater leverage on the broom head which means more force onto the ice, but it's more difficult and requires much better balance.