What is actually happening?

Wondering if the nipple stimulation was keeping it going I lay my arms by my side but it just kept on, and twice more.

The sensations you described are really quite normal for men experiencing prostate orgasms for the first time. Yes, it is very likely your nipple stimulation helped maintain the high arousal necessary to sustain the orgasm.

There was never a hint of an orgasm coming at all, but having got to such a height I just had to make myself relax with an ejaculation by hand.

Here I will somewhat disagree with you. While what you experienced may not have emulated the sensations of a penis based ejaculatory orgasm it certainly had the characteristics of prostate based orgasms, (please see the thread What's an Orgasm?). I think you need to understand prostate based orgasms are qualitatively different from penile based orgasms, while they may exhibit similar levels of pleasure and sensations there are also distinct differences which you have already experienced.

Can anyone try and explain why my body reacts this way, what signals the limbs to start all of the involuntary movements?

You are stimulating your body in a new manner, doesn't it make sense that your body would respond in a new manner as well? As far as I know there has been very little (if any) scientific study done regarding the arousal buildup, effects and trigger mechanisms for prostate based orgasms. This is an area of human sexuality which is woefully under-informed. We Aneros users are a de facto test group for providing this information, albeit anecdotally.

It’s amazing and I still can’t quite believe what is going on.

BELIEVE IT! You are not having hallucinations, your experiences are very real.

Was it a backward step to finish myself off manually? I would have found it very difficult not to have done to be honest.

There is some debate about this, IMHO, I don't believe it will necessarily set you "...backward..." but I don't believe it will allow you to progress in your learning as rapidly as you might otherwise. Please read @Cockadoodle ’s thread Penis, NOT and @rook 's thread Whole Body/Whole Mind - the mental side & "zoneros" for a little insight. It might also help you to read @neros 's post The truth behind how to "rewire".
Good Vibes to You !

Thanks for the very helpful reply. Makes sense to me seeing it from that perspective and thinking about the points you make. So if that is a prostate orgasm or something like it, I need to just enjoy it in a different way from a normal penis based orgasm. Does it ever reach the point where it will with time and patience reach the same climax as a penis based orgasm and result in ejaculation? I will read the topics that you have linked too with great interest.

Onwards and upwards as they say!

Does it ever reach the point where it will with time and patience reach the same climax as a penis based orgasm and result in ejaculation?

For a few men, HFWO's do occur which mimic traditional penile based ejaculatory orgasms. For most of these men it precludes them from getting to the Super-O level due to significant loss of arousal from their refractory period kicking in. Remember ejaculation and orgasm are separate events and there are differing hormonal responses to each. It is possible to train yourself to have HFWO's but is that really what you want to rewire yourself for?

For most men, a prostate based orgasms do not result in ejaculation if penile stimulation is avoided. This is what makes sustained multiple orgasm Anerosessions and Super-O's possible.
Good Vibes to You !

“It is possible to train yourself to have HFWO's but is that really what you want to rewire yourself for?”

No not at all. I am wanting to explore all of that is possible with the use of the Helix alone. A whole new world of pleasure has been revealed that I did not know existed. It makes sense to take advantage of this. I am inspired. Traditional solo sessions whilst enjoyable sometimes felt a bit mundane and so-so.
Thanks again for your advice.