The mind fuck is one of the most powerful tools of the D/s dynamic. For those unfamiliar with the term, a mind fuck is like the manifestation of irony. It can take form in several ways, often along the lines of a "too much of a good thing," "be careful what you wish for," or "remember who is in control" type of a lesson/activity. From my personal experiences, the mind fuck has the potential to push a sub to the absolute deepest regions of subspace, often by twisting desires, fantasies, and fetishes into something that brings a mix of pleasure or hope with mental or physical anguish.
It is probably most easily described with some examples:
-A sub begs to play with his penis. Grant him permission but under the stipulations that he cannot cum and he cannot stop until you give him permission to stop (at this point it's too late to back out). Options at this point may include forcing him to keep going until he cums without permission and punishing him, forcing him to keep going until he is throbbing and raw and denying him immediately before orgasm. If he is smart he will think twice about asking to touch himself again.
-A sub earns and orgasm. Order him to masturbate/cum in a humiliating manner and/or force him to do something unpleasant with his seed after he cums.
-A sub in chastity earns his day of release. On that day release his chastity device but lock his hands behind his back and keep them that way until midnight. After midnight return the chastity device to its locked state. If/when he complains tell him that he got his release and penalize him an extra week or two until his next release reminding him that chastity release doesn't mean orgasm.
-A sissy wishes to be forced to dress up when you haven't ordered it. Allow him to dress but then force him to perform activities that may be beyond his comfort zone while dressed such as ordering the pizza and making him answer the door, having him go fill the car up with gas or pick up something at the store, having him get the mail, do yard work, etc.
-Give your sub a task and tell him you are going to grade his work. Promise a great reward if he gets an "A." Inspect his work thoroughly. Even if it's A quality work, give him an A- and tell him you are one of those teachers who doesn't believe in A's since there's always room for improvement.
-Ask your sub if he wants to play a game (telling him what game you are going to play). If he wins the game he gets a reward/privilege. If he loses, he will get punished or lose a privilege. Stack the game in your favor. e.g. Tell him you are going to play poker, if he agrees then you deal him 5 cards and you get 20 (it's up to you if you want to let him draw or not, allow yourself to draw as many cards as you wish). Tell him you will roll dice to see who gets the higher number, let him roll 1 dice and on your turn roll 5 die (requiring him to roll a 6 and you to roll five 1's to win). Continue for as many rounds as you see fit, with each round counting as a win or loss. Ties should always go to the Mistress.
I know this might seem kind of cruel but it is truly an intense experience.