In the midst of his ecstatic moving and beatific smiling, Marcus gasped unexpectedly as Tonks changed her movements subtly. The new motions seemed purposeful, and the Ranger thought he saw a flare of resolution in the young witch's eyes. Whatever new thing she was doing, it felt amazing, and more than amazing.
After the first few times, Marcus had developed some facility at holding back when necessary, and generally decided at what point he would acheive his release. What Tonks was doing now was making all of that very difficult. Every twitch she made seemed calculated to bring him closer to his climax. His only 'consolation' was that the same actions seemed to be acheiving the same result in her.