When it comes to Dallas' style of discipline, there is always a method, to the madness. Even when it comes down to teaching a Demi-Goddess, a good lesson! The conveyor belt last time, was a broad, all bottom encompassing burn. The thick ruler strap, took that initial burn and intensified the sting, so that she felt two types of stinging burn, on her still hot bottom. This strap was the first implement he used on Sierra Salem! It was not the hand she got first, but this implement, as she stood with her hands against the door, with her bottomed thrusted out. I think my no-nonsense approach that first night, made her feel that she had found a disciplinarian who would challenge her and yes...put her in her place. Just like he does in this scene, with Wonder Woman.