My sub describes it as the point where his brain finally shuts completely off. Not just worries and concerns disappearing but also thoughts like "wow this feels good" and "I hope this happens next" vanish and he really has no thoughts at all and is "just emotion and nerves."
I can't tell you better than that, since I've never experienced it myself.
It's not something we "achieve" on purpose; it just sometimes happens when we're feeling a certain way. Oh, but do I get a rush when he gets like that and that's when I know I can do anything at all but I tend to just continue doing what I was doing to make it last.
Also, and I don't know if this is true for anyone else, but this was something that did not happen for him for the first several years we were together. For him, the level of trust necessary to let go like that is very high.