You want to put your slave away for a while. Maybe as a punishment, maybe you're just busy and don't want him to bother you, or maybe it's just how you play. Either way, it's time for a time out. Go to the corner, be silent, and be still.
Body movement
The slave should be completely still. There are many ways to achieve physical restraint, from handcuffs to duct tape to the Academy-Men-style chair. Of all the options, I like an invisible setup.
Have the slave sit at attention. Back straight, neck straight, eyes front. Palms down, flat on the laps. Legs shoulder width apart, toes front. Right angles at the ankles, the knees, and the hips. Properly practiced, the slave should be able to hold this position for hours.
Aim a motion sensor at the slave and link the sensor to some electricity. Whenever the slave moves, electricity is turned on. This causes the slave pain and makes the slave want to move even more, but the only way for the electricity to stop is to stay still despite the pain. When the pain stops, the slave might be stuck in an awkward position due to cringing from the pain. The slave has to then hold this awkward position or risk incurring another jolt of electricity.
I like this setup because it requires the slave's cooperation. The slave has to hold himself still through his own willpower. Further, if the wires are hidden well, it'll look like the slave is voluntarily sitting still with no coersion at all. The slave will be able to get away with minor movements, such as silently flexing a sore muscle or wiggling a finger, but motion sensors can be quite sensitive and the slave won't be able to get away with much. What little he can do makes the body movements he cannot do even more tantalizing, thus frustrating, thus tormenting. And that's the kind of suffering I like.
To make the situation more unfair, put itching powder on the slave, or have the electricity turn on at random regardless of the motion sensor, or walk in front of the sensor and have the slave pay for your movement. Or turn up the heat, and the slave will have to avoid getting sweat in his eyes without being able to wipe it away.
Sound and speech
The slave is not allowed to make any noise during the time out. Not even a cough. Not even breathing too loudly. Again, I like a sensor for this. A directional microphone trained on the slave and hooked up to electricity.
Alternatively, the slave is required to recite a mantra over and over. It could be as simple as "I'm sorry, sir. Thank you, sir." Or it could be something long and ridiculous and difficult to remember. A speech-to-text software will transcribe the slave's words and turn on electricity whenever the slave's speech deviates from the pre-approved script. If the slave shouts in response to the jolt and the software transcribes it into "Aah!", that's another mistake and another jolt. If the software makes a transcription mistake, too bad.
For extra credit, have the slave count his mantra. "One, sir, sorry, sir. Two, sir, sorry, sir." Maybe have the slave play Fizz Buzz with his mantra. On multiples of three, say "thank you, sir" instead. On multiples of five, say "you're my lord, sir". On multiples of fifteen, say both. "Fifteen, sir, you're my lord, sir, thank you, sir."
It's very easy to lose count. If the slave stays silent for too long because he's unsure where he's at, he gets shocked anyway.
Eye movement
Eye tracking technology exists. The slave puts on a pair of glasses with cameras and the software will be able to tell what the slave is looking at. I don't know if these glasses are produced commercially, and in any case the software will need to be heavily modified, so the ideas below might be difficult to carry out. But we can fantasize.
Draw a circle on the wall, and if the slave looks away from the circle, he gets shocked. The smaller the circle, the more disciplined the slave has to be with his eyes.
If the slave is reciting numbered mantras, draw a few circles and require the slave to switch circles in between counts. If he's in enforced silence, we still can't allow his mind to idle and wander. Draw a keypad on the wall and give the slave math problems. Have him type out the answers using eye movement. If he gets an answer wrong, he gets shocked. Remember he can't move his head, nor do the math out loud, or he gets shocked. He can only move his eye balls.
Control the slave's blinking as well. In easy mode, make the computer beep once every 10 seconds. Every time the computer beeps, the slave has a one-second window to blink once. If he doesn't blink or blinks more than once, he gets shocked. If he blinks outside the window, he gets shocked. To increase the difficulty, make the beeping interval random. Make the upper end of the random interval uncomfortably long, like 30 or even 60 seconds. Or, stop the computer from beeping and require the slave to keep pace on his own. If he gets the timing wrong, he gets shocked (and maybe resume the beeping for a minute to allow the slave to find pace again).
I'm not a doctor. Not blinking for a prolonged period of time could be bad for the eyes. This is a fantasy only.
Breathing
The slave could be hooked up to a ventilator. In the same vein, I prefer a machine that monitors the slave's breathing and punishes him when he breathes out of turn, rather than a machine that does the breathing for him. This section leans heavily into fantasy side of the scale. Also, never do breath play alone.
If the slave isn't reciting a mantra, require him to follow a breathing pattern. Breathe in for 4 seconds, hold for 7 seconds, breathe out for 8 seconds. To increase difficulty, require the slave to hold for longer. Holding with a full lung of air is easier than holding with an empty lung, but it still gets challenging quickly.
Remember the slave can't let his chest heave too quickly or the motion sensor will trigger. The slave can't breathe too loudly or the sound trigger will trigger. The slave can't forget to do math with his eyes or he'll get shocked. All the mental juggling will for sure occupy the slave's entire mind and make the time out an exhausting exercise, even though all he's doing is sitting still.
Facial expression
A slave doesn't get any freedom during the time out. He can't move, he can't look away, he can't even think for himself. On top of all that, require the slave to stay smiling throughout. I don't know how this can be enforced technologically, so let's say we have someone watching over the slave who can turn on the electricity manually.
Thing about smiling is, it fools your brain into thinking you're enjoying whatever you're doing. The slave might just come out of this ordeal feeling good and satisfied, which is fine by me. Gives me another excuse to subject him to the time out again.