Then I'll mention the 3 basics here: safe, healthy, consensual
safe like: that you know what you are doing, have a plan B, be aware of what can go wrong, consider the consequences of your actions
healthy like: not under the influence of alcohol and other drugs that can override your perception and sense of pain, be aware of physical limitations and risks (e.g. breath play)
also consider the emotional influences, when you are euphoric you can perceive your limits differently
but the opposite, collapsing, can also happen, which is why there is a safe word or aftercare
I shouldn't have to explain
consensual, but it's clear that both should understand what they want to do and consciously agree to it
Of course you can add more information to these words, so it is only intended as initial information and is not complete
As
@subzzzero has already written, aftercare is about supporting the other person physically and emotionally, feelings can be very overwhelming and it's important not to be left alone with them. For me, aftercare is also about talking about it afterwards to better understand how the person felt, what was good and what wasn't so good. Reviewing the session once again.
You can never have enough knowledge and understanding.