An endorphin high is exactly what its called. A high of hormones, mostly endorphins. During a session, whether it involves impact play or not, our bodies produce an incredible amount of hormones.
The high that comes from the endorphins is usually more often experienced by submissives or slaves. Mostly during the session itself. It is also called flying or floating and refers to the submissive part kind of drifting away. It can really feel like you’re on drugs, even tho you are not.
This happens because our body usually is not used to impact on it as intense as it can be the case while practicing BDSM. While it can happen that you’re „getting used to it“ you will probably still live through such a high every now and then.
It shows by the submissive not being really „there“ anymore. They may be physically there, but mentally have gone to another world, also called sub space. In sub space, you will realize that your pain tolerance will change (due to the hormonal high).
After a session or an impact play ends, the submissive part can fall into what is called sub drop tho. This refers to the endorphins rapidly going down to their normal levels, which can result in many different ways. Read more about this under sub space and sub drop.
Dominants can experience an endorphin high as well as a Domdrop too. But, since dom is leading, you as a submissive part will probably not realize when it happens.
Also, when doing an impact play or session with an experienced dominant who knows, what they are doing, at least a Domdrop should not happen often to not at all.
Yet, an endorphin high can also catch dom off guard. It should never affect them in the same way as it affects a submissive tho. Imagine your dominant drifting mentally away. That is scary as hell and you should always be able to count on the fact that they are going to end the session whenever they feel like things are getting out of their control.