Similar cervical spine accidents paralyse 98 per cent of recipients and destroy the lives of a few more when the delicate but mandatory surgical procedure goes awry.
The vertebrae's explosion released the pressure on Nicholson's spinal cord which enabled him to get up, walk around and avoid the usual devastating symptoms. Eight weeks on, he is sitting and eating without a neck brace and should be walking freely in a week.
In short, his recovery to this point has been miraculous - a spine-tingling story.
"Bloody horse" was Nicholson's first thought as he embarked on the split-second journey from saddle to turf.
"You know before you hit the ground how much it's going to hurt," he says. "I had my leg trapped but got that out before the horse rolled over. I thought I'd broken my shoulder because that's where most of the pain was. Then I realised I could move it all right, but the tingling down my arms got worse.
"I knew I couldn't ride the next horse and called for the doctor. The idea was to get some more painkillers, but I was moving about a lot and changed from riding to normal clothes. The doctor suggested I go to the hospital for an X-ray."
A CT scan unleashed a cacophony of medical alarm bells. Nicholson was immediately strapped to a bed, then sent on an hour's drive to John Radcliffe Hospital, listed by Britain's National Health Service as providing "acute medical and surgical services, trauma and intensive care".
Nicholson underwent eight hours of surgery. It was led by Jeremy Reynolds who, according to Nicholson's wife Wiggy, said her husband had "more chance of winning the lottery with a single ticket" than recovering with such minimal damage. The shattered vertebrae was replaced with an implant. The other bones in the spinal column were re-aligned.
"They couldn't believe he was able to move," Wiggy says. "That's where the surgeon's skills came in. He had a fully mobile patient going to theatre knowing that when he came out, there was a high risk he would be paralysed.
"That's huge pressure. Normally patients are already paralysed and the damage is irreversible. Mind you, Andrew had no choice. He would have become a tetraplegic if they hadn't done the surgery."
Nicholson will return for another X-ray in four weeks to make sure everything is in place. His ambition is to get back on the horses, so to speak, after the winter break.
"I can't see that being a problem," he says. "I know why I landed on the ground. I was going fast because the plan was to win the class so I could ride my second horse at a slower speed because it was entered for Burghley. You know the risks."
Nicholson hasn't ruled out his increasingly slim chances of selection for a record eighth Olympic Games.
However, he acknowledges such a matter is trivial compared to his wider health.
"I go walking a lot and a specialist neuro physio works on my nerves to get back to a normal range of movement and function. It should be a case of letting everything knit and fuse over time. I'm focused on riding again but we're not in any rush this time of year."
Nicholson's famed tenacity has paid dividends.
"When the first CT scan came through, the hospital explained how bad it was to Wiggy. [As a trained vet] she better understands that sort of stuff and the complications involved.
"They explained it to me, but I had [earlier] walked into the ambulance and could still move my hands, feet and legs so I wouldn't listen. I'm pretty stubborn. In my mind, I was going to come out all right.
"I've ended up with a plate on the inside of my spine which means I can't talk very loud. Wiggy knows if she steps far enough back she can't hear me. Maybe the surgeon did that on purpose," Nicholson laughs.
"Luckily for me, Andrew never thought he was hanging over the edge," Wiggy says. "His mental strength is phenomenal. He is now fully mobile, albeit with relatively minor nerve damage.
"First he has to recover on a normal human being level and then return to being an international event rider, but he's following a good line of progression and becoming increasingly difficult to manage ... which is a good sign. I'm just so happy he's fine."
Nicholson's recovery has been helped by Britain's Injured Jockeys Fund, founded in 1964 and which has spent over 18 million helping more than 1000 former jockeys.
"They have given us great guidance about where to go for specialist care during this phase," Wiggy says. "Andrew rode in a charity race at National Hunt Jump racing festival in March and they're repaying the favour.
"He's getting no professional athlete support from anyone in any other way.
"The racing world has a massive knowledge on this because they have seen more high-speed impact neck traumas than most other sports."
The Nicholsons had no insurance for such an accident, but the success of their Wiltshire operation means their livelihood is not under threat. Their staff can still prepare and look after the stables.
"Twenty-five years ago, I opted out [of getting insured] because it was too expensive to keep up the payments," Nicholson says.
"Basically I had to do what I've done now to make it worthwhile, but we'll get by. We'll sell a few horses. I realise I've been lucky and the support from all over the world has been touching."
