The Australian Test captain says he instructed wicketkeeper Alex Carey to throw down Bairstow's stumps in a dismissal that sparked a furore.
While Carey has worn the brunt of mainly English criticism for the dismissal, Cummins has revealed for the first time that the plan was his.
Cummins, in the Prime Video documentary The Test - Season Three, which premieres on Friday, says he noticed Bairstow walking out of his crease repeatedly during the Lord's Test in June-July last year.
"I saw him do it a couple of balls in a row," Cummins said in the documentary.
"Cam Green was bowling and bowled a bouncer and he ducked underneath it and then just walked out of his crease.
"So I just said to Kez (Carey) the ball before, I said 'Kez, just have a throw'."
Carey knew the final delivery of the Green over, on day four of the second Ashes Test, would be short.
"I knew it was going to be a bouncer, so more than likely the ball was coming my way," Carey said.
"Short ball, stayed a bit low, landed a foot in front of me. So I picked it up on the half-volley ... turned the body to throw it - it was on target, it didn't look like missing."
Carey's throw hit the stumps as Bairstow walked down the pitch.
The stumping dismissal of England's Jonny Bairstow added plenty of spice to the Ashes series. (AP PHOTO)
On video review he was given out, prompting scenes that Australian fielder Matt Renshaw, who was substituting for an injured Nathan Lyon, described as "absolute carnage".
Bairstow's replacement Stuart Broad arrived at the pitch and sprayed Cummins and Carey, telling the wicketkeeper: "'That is all you will be remembered for."
At the lunch break soon after Bairstow's dismissal, Australian players were abused by Lord's members inside the Long Room as they made their way to the dressing room.
Cummins: "It was like we'd ripped the soul of out them ... absolutely, yeah, people stepped over the line.''
Usman Khawaja: "One of them ... (was) spraying me. I was like 'nup, you can't be saying that stuff'. He said 'oh, I can say whatever I effing want', like a sense of entitlement almost."
Marnus Labuschagne: "One of them was foaming at the mouth. A bloke hit Bull (David Warner) when he went up the stairs."
Reserve batsman Marcus Harris: "They were kicking off over the fence. I was like 'you write the rules you f***ing  idiots, it's not our fault'."
Cummins was shocked at the backlash to a dismissal he said was "just clear-cut, it was out".
England claimed the Australians had broken the spirit of cricket because Bairstow wasn't attempting a run.
"People use the spirit of cricket in whichever way it suits them on that day," Cummins said.
"You do something, you expect a certain outcome. That (dismissal) was, like, I thought, relatively vanilla. And it just blew up."
Lyon said the spirit of cricket argument was "a load of crap".
"It's a stumping. It's out every day of the week," he said.
Khawaja also rejected claims the Australians had breached the spirit of the game.
"Everyone has got their own spirit of cricket," he said.
"But the laws of the game are the laws of the game. If it's within the laws of the game, it can't be against the spirit of cricket."
-1°C