He keeps an eye on her as she begins to struggle, knowing she'll probably need a break soon. The point of this is not to hurt her of course. There's nothing to be gained from her pulling a muscle or straining herself, but he also wants to see if she has the discipline to know when she's taken on more than she can handle and the strength to admit as much.

"Careful," he says. "Use the muscles in your arms, not your back."