McCullum took up his post just a few weeks ago with a promise to bring a fresh, positive outlook to a side that had failed to win any of its previous five series and has immediately succeeded in changing the atmosphere.
He has received rave reviews from his charges for the mood of levity and optimism he has brought to the group, with the on-field results taking a sharp upturn too.
After an impressive win over McCullum's fellow New Zealanders at Lord's, England produced an astonishing run chase in the second Test, clearing a target of 299 in just 50 overs.
Just a year earlier an England side under the guidance of Chris Silverwood settled for a draw after refusing to take on a target of 273 in 75 against the same opponents.
"There's no doubt that Baz has had an impact already," said Broad, who has previously played for England under Peter Moores, Andy Flower, Trevor Bayliss and Silverwood
"It does feel really fresh and exciting in the changing room. It's a very positive language. It's very forward-thinking, all about how to move this game forward.
"This is no dig but at tea, when we were four down with the game in the balance slightly, I've certainly been in changing rooms in the past where that would be shut up shop time.
"Baz's team talk was very much 'let's attack the danger, let's run towards the danger' so every part of your mind is about going for this win.
"It was never really a case of 'if we lose one we might shut up shop'. It was always 'we're going to win' and if it doesn't work, don't worry about it."
McCullum vowed to strip out the 'noise' around the England side and Broad has been impressed by his ability to combine his nous for the game with a simplified mission statement.
"I don't think he's spoken particularly deeply, his whole mantra is about enjoyment and fun. The energy is: how good's Test cricket? How good's this ground? What can we get out of today?" said the 35-year-old seamer.
"He looks like a guy who has a cricket brain that is working all the time, he's thinking how we can change the game.
"It's not just praising guys who get a hundred, it's tiny little things, bits of fielding, momentum changes in the game. He will bring attention to that."
McCullum's positive impact was only a platform at Trent Bridge, though. It was Jonny Bairstow who turned his words into unforgettable deeds, smashing a 77-ball century in a game changing 179-run partnership with Ben Stokes.
Bairstow's knock was an instant classic and good enough to leave a 153-cap veteran like Broad in disbelief.
"I didn't quite expect to see what I saw from Jonny. It was the most outrageous hour I've seen in Test cricket from a partnership. It was just exhilarating, astonishing.
"That striking...only a handful of players in the world can do that. Jonny is obviously in that group."
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