Magnussen doesn't flick John's face. He doesn't touch either of them. He finds it much more amusing to tell John to flick Sherlock's face. Because then it's John hurting Sherlock to keep Mary safe, and Sherlock letting him. That's real leverage.
The commands start with face flicking. They get worse.
John/Sherlock and Magnussen, Non-con
Magnussen doesn't flick John's face. He doesn't touch either of them. He finds it much more amusing to tell John to flick Sherlock's face. Because then it's John hurting Sherlock to keep Mary safe, and Sherlock letting him. That's real leverage.
The commands start with face flicking. They get worse.