Strange spasming feeling?

I have had that happen as well. I will start with uncontrolled changes in breathing patterns. That will happen along with uncontrolled ab spasms, and thrusting. I have at times stopped breathing, meaning not inhaling/exhaling and am unable too if I try.
I have felt the feeling of sinking into my bed as if becoming totally relaxed and i become very hot. I believe I black out at some point because I dont remember what happens after I peak.
But its a dry orgasm and accompanies an anal orgasms where, like you said, the anus is in a spasm, but for me its a very specific rythm and until I black out, i cant control it or miss a beat or disrupt it as it controls itself. As this is happening, this is where my breath begets me and I believe I pass out.
I come too, and it starts over again. I have had this cycle for 4-5 hours.
I am normally in a pool of precum.
The next few days, I will feel "ghosting" where I cant do anything sitting down because it causes me to start the above mentioned cycle even without the aneros in. First time I experienced "ghosting" was a few years after i got the aneros and never got anything out of it. Then one day I fell asleep and WOW.
On my way to the mall that following afternoon, I was in my car and had to stop constantly as I kept having those anal orgasms , as the nite before was my first one. I actually started to cry becuase I though something was wrong. it took me 3 hours to get to a mall 30 mins away. And when I arrived, i was soooo dehydrated and my lap was wet with a large stain of pre cum that ran about 3 inches down my pant leg.
So, I could not go into the mall, and had to lay on the back seat for 3 hours and try to drive home again, and thank god I made it.
Thats my true story

Wow that does not sound good. You should not be blacking out maybe you need to see a doctor about it.

women who have true orgasms are known to black out as well. Due to the amount of endorphines released during the orgasm phases.