A young fan's immediate reaction to the underarm delivery was enough to tell Greg Chappell he'd overstepped the line.
A documentary that unpicks every aspect of the incident still regarded as the darkest act in the trans-Tasman sporting rivalry will go to air in New Zealand next week.
Trevor Chappell's rollout to tailender Brian McKechnie at the MCG happened 38 years ago this Friday.
To mark the occasion, a collaborative broadcasting effort from both sides of the Tasman interviewed the key players from each team.
Interviewer Eric Young said the exercise proved cathartic for both Chappell siblings and most obviously for an emotional Greg, the underarm mastermind.
Young said he could hear an exhalation of relief from the former Australian captain when he was pitched the interview.
"Some of the things Greg has to say about his little brother will touch anyone who has a little brother," Young told AAP.
"There are some quite moving moments.
"Greg's very honest about his emotional wellbeing on that day. His mental and physical exhaustion, is the origin story of underarm."
Greg instructed Trevor to roll the final delivery - legal at the time - to deny McKechnie any chance of hitting a match-tying six.
McKechnie blocked the ball and flung his bat in disgust.
It didn't take long for recriminations to begin.
As spectators invaded the ground, a young girl tugged on Greg Chappell's shirtsleeve, he revealed in the documentary.
"She was looking up at me and she said 'you cheated'. At that moment I thought that this might be a bit bigger than I'd even imagined."
Australian vice captain Kim Hughes and opener Graeme Wood were also interviewed.
The Seven Network will show the documentary in Australia at an unspecified date.
Hughes drew comparisons between the underarm and the Cape Town sandpaper scandal that rocked Australian cricket last year, believing both stemmed from intense pressure to win.
"Hughes doesn't lay blame of any kind at the feet of Greg Chappell," Young said.
For Kiwis, the underarm has had 38 years of positive spinoffs, and not just for the sporting banter moral high ground they've been gifted.
The captain on February 1, 1981 was Geoff Howarth, who was angered at first but then noted how interest in his team subsequently exploded.
"Geoff said it was the greatest thing to happen to New Zealand cricket," Young said.
"We call it the ball that changed cricket."
