Hi Sue,
I'm not Cathe nor have I played her on TV, but I have her slow & heavy series as well as many others. I think the whole point of putting your feet up on the step is to help to keep your back from arching. If you're like me unless I position myself just right I have trouble finding room to put my feet up on the step. Just make sure you concentrate on keeping your back flat, like when doing abs.
Kay