Then there is the true anal orgasm... the most amazing and almost indescribable internal explosion. Maybe like an explosion underwater. You feel the concussion then the aftershocks and finally the ripples. You can barely stand up and rational thought is out of the question.
I have only experienced an anal orgasm once, but my experience was almost exactly as gemdd described.
My wife and lover was travelling, and as a "present" to me, she left me a cassette tape with the most beautiful recording expressing her love - and ending with explicit instructions that I was to give myself an erotic enema at exactly 8:00pm that night so she could think about me and watch me with her "minds eye".
I was so turned on by the invitation! At the appointed time I made all of the preparations, and naked, I bent over our bed as I had been instructed and reached behind to "thoroughly lubricate" myself per her orders. After penetrating myself with first one, then two fingers - and knowing she was "watching" me from afar, it happened....
I experienced an explosion deep inside of my anus followed by waves of pleasure - like the ripples in a pond that follow the splash of a thrown pebble, except that the "splash" emulated from my anus and the ripples flowed outward from there through my entire body. The orgasm was so complete satisfying that I felt no need to ejaculate.
After the initial experience quieted down, I completed the enema and then soaked for an hour in a warm bathtub while the remaining ripples continued to flow.
It was amazing!