YAY!!!! This one is ALL ME! I am a BDSM porn star known for being one of the hardest bottoms in the industry and yes I have experienced this as well as “rope space”
Basically there is a point where the mix of extreme pain and pleasure force you into a shock like state where you completely let go of control or fear and the desire to resist and its really amazing and almost out of body experience. This is one of the reasons we us spotters here for safety. The first time I experienced sub space was when I was going through slave training and was doing breath play being chocked out and when I woke up from it I cried and felt like I had been reborn.
After care is also a very important step because of sub space. the dom really needs to make sure the sub returns to normal before leaving. gently stroking the hair or rubbing the subs back and reassuring them that everything is fine and its ok to go back to being your no sub personality.
I hope this was helpful