The struggling Bluebirds pulled off a surprise result by upsetting promotion-chasing Northampton thanks to Aaron Amadi-Holloway's second-half header.
"We were up against a form team and a really good team and we had to work hard for it," said Cooper.
"It was a tough game and no-one expected us to win here but I thought we competed well and we defended fantastically.
"We always carried a threat and we put our bodies on the line - which should be a given - and I think with the team I picked that was the main ingredient.
"Northampton have been playing with two midfield players up front and no out-and-out striker so I wanted to play a really mobile back three to deal with their threat and their pace in behind.
"I picked players that have pace and mobility and would be quick on the turn and that was the reasoning behind and I was really pleased with how it turned out.
"It's an old adage but when you come to an away game, you want to silence the crowd and get them turning on their team and I think we managed to do that quite early on.
"We started really well and when you do that, you give yourself a chance to win."