Though she thinks little of it now, Meredith enjoys being the one in control here, one small pocket of her life where she gets to call the shots and please him all at once, leaving her more smug than she probably has a right to be. He hardly moves, but it's amplified by proximity, and she picks up her speed just a little in answer — there's a line between prolonging this and doing so unnecessarily, and she doesn't want to torture him, just to take her time, feel him so perfectly on edge under her.