autopilot breathing is pretty cool, its one less thing to think about. A bit difficult to get running once you're aware its no longer active, though.

Favourite breathing technique: hyperventilating

What effect does it bestow: anxiety, heart racing, lightheadedness, passing out

You now breathe manually

I use Total Concentration Breathing which enhances my physical abilities to supernatural levels and allows me to slay demons.

Max/max/10, followed by

90%/max/10

80%/max/10, etc.

Point is to inhale slightly less each time, hold as long as you can, then slowly breathe out. Works wonders for helping me fall asleep or just generally relaxed when I'm anxious or not sleepy.

4-5 / 0-1 / 4-5 / 0-1, repeated for at least 1 minute, better 5 minutes or more. (not too deep or light-headedness can happen)

This is resonance frequency breathing (at round about 0,1 Hz) which maximizes heart rate variability and activates the baroreflex (blood pressure). It's usually used with a HRV biofeedback device to find the best individual breathing frequency, but it's not strictly necessary. Just find the speed around these marks that feels good (without feeling like you're out of breath or getting dizzy). Breath into your belly.

There's tons of studies for its effects, but mostly it activates your vagal control, and thus tones down anxiety, can help with depression and PTSD symptoms when done regularly, and also helps with high blood pressure, can make recovery after heart surgery easier, is used in professional sports, may help with IBS etc. Look for papers on "HRV biofeedback effects", if you're interested.

The most immediate effect I personally have is feeling both more relaxed and more alert, similar to after meditation.

Was expecting a lot more Demon Slayer references here :(

3/1/4-5/2-4

for added stability and/or situational awareness

when you are choaking and can't breathe in your mouth, use your nose.

I mean, box breathing like you described works because you're depriving yourself of oxygen briefly lol. Causes blood pressure and anxiety and stuff to lower.

Just paying attention to my breath, and trying to breathe about 5% deeper than I would unconsciously.

If I do that a few minutes a day for a few days, I enter a state of relaxation that’s off the charts.