Dark "tests" him from time to time. Checking if his most useful tool is functional, whether that be physically or mentally. Normally he'd instruct him beforehand but he keeps him on his toes by saying them as he goes, he likes seeing him hang off of every word he says
When the bottom orders the top to go harder to the point of begging, squirming for that orgasm they can't seem to reach, as the top goes faster, deeper, they get worried for a split second but keep going because the bottom insists, demands even, my bread and butter