Some calls are a little more memorable than others. If Packie Bonnar had decided to dive to his left instead of right during the penalty shoot out in Genoa the cult of St Jack might never have lasted as long as it did.

Then there are those calls that effect lives, and deaths, and resound across the generations. It was this kind of a call that was made last Saturday at Epsom. A life was lost, thankfully not human, but it may have been one of the more expensive decisions ever taken in sport.

Just before 4.30, just as the most exciting finish to the Derby in years was beginning to unfurl, the Aidan O'Brientrained second favourite Horatio Nelson shuddered to a painful halt. His jockey, Kieren Fallon, dismounted quickly, and as he was whispering words of solace to his distressed ally, Sir Percy lunged for a gap on the rails and immortality by a short head.

By the time the winner had been pulled up there were screens erected around Horatio, his short years of brilliance almost over.

There are dozens of big reasons why O'Brien is Ireland's most successful international sportsman, but it's the small things that tell the true story.

For instance, in Paris last October he saddled Scorpion, an excitable son of Monjeu, for the Prix de l'Arc de Triomphe. He wasn't strongly fancied for the race, and there were few watching him as he left the parade ring, a real nuisance, kicking and stropping . . . burning the precious energy he needed for the race.

O'Brien, as usual, was not far from his team in their hour of need. He gently took the reins, soothed the horse, and walked him a couple of hundred reassuring metres before releasing him to canter to the start.

Whatever he did worked.

When he released Scorpion the colt was nearly as calm as a fairground pony. He knows the minds of his horses as much as he knows their ability to gallop. And he worries about them all equally.

His difficulty is that his name is often mentioned in sentences that also contain the words 'Coolmore, billionaire and Magnier'. The Coolmore brand signifies success, wealth, and financial opulence.

The O'Brien brand is built around caring sensitively for horses. This sensitivity is what brought O'Brien to the start of the Derby last week. He wanted to make sure he was personally managing the welfare of his four entries in the moments before the off, while his rivals looked on remotely from a mile and a half away.

What happened next, 'the call', has been the source of animated and sometimes misinformed debate ever since.

Here are the visuals. Horatio Nelson canters to the start while the television cameras focus on Linda's Lad who had turned mule in front of the stands.

Kieren Fallon, who had chosen to ride Horatio following a sparkling gallop earlier that week, grows concerned that his mount is a bit stiff in his action on the way to post. He trots the horse around so it can be inspected by the trainer and the racecourse vet. Fallon was not concerned enough to get off the horse.

After some discussion O'Brien appears to signify that all is well to his jockey, and hurries past a television interviewer back to his car. The BBC camera mounted on the stalls zooms in on Fallon, who seems to look slightly agitated and uptight, although nothing can be heard of what is being said. The stalls open, and the race, which ends so tragically for Horatio Nelson, gets underway.

Rumour emerged as soon the contest was over. The BBC presenters began reading emails from lip readers and armchair vets who claimed Fallon had told O'Brien that the horse was crocked, pleaded for withdrawal and didn't want to take part in the race.

One rumour claimed O'Brien was telling Fallon that his job was just "to ride the effin horse."

To summarise the rumours.

Aidan O'Brien callously overruled his jockey and allowed an injured horse to take part in a rough classic, on firm ground, on one of the quirkiest and unbalanced racecourses in the world. And all this in the hope of boosting Horatio Nelson's already considerable stud value? This, however, is a theory that fails on every level.

If this was the case then why didn't Fallon pull him up after a furlong if he disagreed with the decision to run? Why did the racecourse vet, Jenny Hall, not overrule his participation? How come the horse raced prominently and had every chance of winning at the point he suffered the injury? Why would his value as a stallion be risked by running him unsound?

It is almost impossible to precisely . . . or even remotely . . .

value the future earnings of a stallion, but Horatio Nelson was potentially very important to Coolmore. By Danehill, he was closely related to dual Derby winner Generous and the brilliant mare, Tryptich. Danehill, an outstanding sire, was the first high profile 'shuttle stallion' . . . he covered mares in Europe and Australia in their respective breeding seasons, thereby creating a double dip revenue stream for Coolmore.

Unluckily killed in a paddock accident in 2003, Danehill has proven to be that holy grail for breeders, a 'sire of sires', and one whose sons are as successful as him at fathering good horses. His most recent high profile stallion is Rock of Gibraltar who covers about 100 mares a year at a nominal cost of about 45,000.

Even if Horatio was to generate half of this revenue, the call to run him last week may well have cost Coolmore up to 20m a year in stud fees.

Speaking in France the day after the race, Fallon was emphatic about what happened at the start. "He was fine when I trotted him up and down a few times, in fact the more I trotted him the better he felt.

"I was happy with him. If I wasn't happy then I wouldn't have gone round on him. He travelled brilliantly on the snaff (bridle) in the race, then we went to go and challenge and it happened. Obviously it must have been one of the uneven bits of ground that you can get on the camber at Epsom."

The extent of the horse's injuries supports the view that it was an independent accident and not the progression of an existing problem. He suffered fractures of the cannon and sesamoid bones and a dislocation of the fetlock joint. It wasn't even remotely possible to save him despite his potential value as a stallion.

Ironically, this was probably the easiest call of the day.

Aidan O'Brien is reluctant to discuss publicly his feelings about last Saturday's events.

It is reasonable to assume that he feels as empty as the box he walks past at Ballydoyle every morning, and that unanswered questions fill his head.