Having to fight against a force with my head and neck to stay upright just does something special for my core and back. That said, I’ve also done the same exercise with the bands attached to my waist and that felt great as well. However, I can’t get anywhere near my maximal exertion with the band attached to waist, due to space constraints and a fear of the band snapping. With the band attached to my head, the maximum possible exertion before my neck gives in is so much less that it’s easier to reach some meaningful resistance.