As the man who had started as Britain's clearest favourite for an Olympic gold medal staggered over the line in seventh place, three flattened hurdles told a graphic story back down his lane, and ahead, McKoy, whose time of 13.12sec had put him clear of a straining second wave of bodies, was already turning to seek him out and embrace him.
The aftermath saw a bizarre reversal of emotions, with Jackson beaming all over his face - 'It couldn't have happened to a nicer guy. He's my best friend. He's like a brother' - and McKoy positively lugubrious. 'I'm more disappointed than I am happy right now because my training partner didn't run one of his better races. We were hoping to do well together.'
McKoy paid tribute to Jackson and their joint coach, Malcolm Arnold, in every other sentence as he picked his way through the post- match conference. 'Without them I would not be here,' he said.
Four years ago his Olympic picture was a very different one as, in the wake of the Ben Johnson scandal, he refused to run for the relay team and left Seoul without permission, earning a two-year suspension. His subsequent admission at the Dubin inquiry that he had taken performance-enhancing drugs under the guidance of Johnson's coach, Charlie Francis, saw him stripped of his Canadian record.
At that point, he was an athlete in the wilderness, but Jackson, whom he had met during the Commonwealth Games of 1986, invited him to come and train with him and Arnold in Cardiff. Thereafter it was onwards and upwards for him, as he strove to put the bad memories of his time in the Francis group behind him. 'It was an unfortunate thing to happen,' he said. 'But I'm a better person for it.'
At the start of the season, McKoy and Jackson had made a pact that they would not race each other until they had to. While Jackson was picking off the leading American challengers, he always maintained that McKoy was the man he feared. His fears proved justified. Having got a worse start than his training partner, Jackson was always straining. He hit the second and third hurdles hard, and clipped the 10th before crossing the line off balance.
Meanwhile Tony Jarrett, having made a poor start, gained ground in lane four to record the joint- third best time of 13.26sec behind Tony Dees, of the United States, second in 13.24. Jarrett's hopes were frustrated, however, as Jack Pierce was given the bronze.
The 400m semi-finals involved a bewildering variety of fortunes for Britain's runners as Quincy Watts, the 19-year-old from the United States, made his case for a gold medal in tomorrow's final by setting an Olympic record of 43.71.
Derek Redmond, running into the best form of his life after four years of injury and operations, sank to the track like a wounded deer half-way down the back straight of the first race, before hopping round to finish, his face distorted with pain and the realisation that it had all gone wrong for him again. For the last 150 metres, he was supported by his father, Jim, who had jumped out of the crowd.
In the stronger, second semi-final, Roger Black, despite his hamstring problem, ran 44.72sec - not enough to get through. Ahead of him, David Grindley, at 19, the second youngest member of the British party, broke Redmond's British record with a time of 44.47.
Mike Conley celebrated his 18.17m on his sixth and final jump in the triple jump with an extravagant somersault by the landing area. It would have been a world record but for the following wind being 2.1m per second, 10cm per second over the legal limit.
In the women's 800m, Ellen Van Langen, of the Netherlands, ran herself into a daze of joy as she overtook the Unified Team runner Lillia Nurutdinova on the inside with 20 metres left to win in 1min 55.54sec.
Linford Christie, on his return to the track following Saturday's 100m victory, John Regis and Marcus Adam all looked in good shape as they went through the first two rounds of the 200m, but Regis is having to have injections in a hamstring strain he picked up on Sunday - not a good omen for a man who plans to run the 400m relay and perhaps the 100m relay as well. Sally Gunnell sailed through to the 400m hurdles final in 53.78sec.
Phylis Smith, of Wolverhampton, gave her four-year-old, Robert, his customary televisual wave before reaching tomorrow's 400m final with a time of 50.40sec, the second fastest of all time by a British woman behind Kathy Cook's 49.43 in 1984. Smith will need to get a lot closer to that mark to have any chance against Marie- Jose Perec, the French world champion, who finished five metres ahead in 49.48.
Matthew Yates, whose place in the team was uncertain until the last minute after a viral infection disrupted his preparations for the Games, ended the day feeling grateful for a few trenchant Scottish oaths from the national director of coaching, Frank Dick.
Twenty minutes before his opening heat in the 1500m, Yates decided he could not face running - 'I walked round the track and I said, 'This isn't for me'.' Dick had other ideas, and Basildon's most loquacious son was persuaded to put himself on the line. He made the pace in a field which included the world champion, Noureddine Morceli, and, when the field streamed past him in the final straight, found an extra burst of energy to hold on to the fifth qualifying place.
For a man who looked a potential medallist after winning the European indoor title in March, it was a desperate performance; but this was a man who had subsequently found himself on a 24- hour drip in a hospital bed, something which has left mental scars which the British team's psychologist, Brian Miller, has been working to heal.
'I've been so depressed lately I've been breaking down and crying,' Yates said. 'Everything seemed to be going wrong. What was scary was I didn't know if I could run. I was scared of failing.'
Kevin McKay, the British trials winner also came through, running 3:37.39 to claim the fifth and last automatic qualifying place in his heat. But for Steve Crabb, drafted into the team when Peter Elliott dropped out, there was only disillusion as a time of 3:41.00 left him well adrift. Crabb, a sections officer for British Gas, may now decide to devote himself more fully to his job. 'I have not been pleased with my running for the last three years,' he said. 'I think this is the final nail in the coffin for me.'