The Belgian midfielder capped another tremendous man-of-the-match performance by superbly drilling an eight-minute free-kick into the unguarded net after the Bluebirds' defensive wall all jumped to a man.
It was an exact replica of the free-kick KDB netted at home to Bournemouth in the Premier League last season.
And Kevin said City's pre-match research on their high-flying Championship opponents had given him an insight into how to exploit the set-piece opportunity.
"They told us before the game (about the wall) and we know all the details," Kevin revealed.
"The wall was not nine metres (away) for me so it was very close so I thought it would be easier to hit it under rather than over!"
With Raheem Sterling's first-half header doubling City's lead, the Blues kept Cardiff at bay after the break though they did lose Leroy Sane who was the victim of one of several X-rated challenges by Cardiff with Joe Bennett eventually seeing red after a late foul on Brahim Diaz.
And with Wednesday's return to Premier League action at home to West Brom in mind, De Bruyne said City took the opportunity after the break to try and stay relatively fresh.
"The second-half was strange because they didn't want to attack us and I think it was a little bit boring for half an hour. But that helped us conserve energy for the next game," Kevin added.
"It was a different playing style to everyone else. The tactics we used were class because Vincent Kompany was always free and we opened up the midfield.
"Every time we pressed them we would play to the man who was marking that guy so we had a lot of space.
"We knew how they played –we just tried to concentrate on our football and they caught us a few times and that is football but obviously Leroy Sane is injured now and hopefully that is not too bad."