Goals from Wesley Sneijder and Klaas Jan Huntelaar, in stoppage time, helped the Dutch overturn a 1-0 deficit and secure a place in the quarter-finals.
"First I changed from 4-3-3 and then we created a lot of chances," he said.
"Then I moved to plan B. I did that in the cooling break - that is a clever way of benefiting from these breaks."
The Dutch barely troubled Mexico keeper Guillermo Ochoa in the first half, with the first drinks break of Brazil 2014 taking place in the 32nd minute.
The 2010 finalists improved after half-time but only after they went a goal behind when Giovani Dos Santos fired in a excellent shot from 25 yards out.
The Netherlands almost hit back straight away when Stefan de Vrij's volley was brilliantly saved by Ochoa at point-blank range.
Van Gaal then brought off lead striker Robin van Persie in the 76th minute for Huntelaar, before giving another tactical team talk during the second drinks break a minute later.
The coach added: "I knew we were going to have this cooling break. We trained for this, with Huntelaar and with Dirk Kuyt up front and long deep balls."
With two minutes left, Sneijder scored with a wonderful half-volley from Huntelaar's nod-down, before the provider converted from the spot after Arjen Robben had been fouled.
|The Netherlands have now turned their last four World Cup games in which they fell behind in regular time (90 minutes, not including extra time) into victories||Wesley Sneijder has scored five goals in his last five World Cup knockout matches for the Netherlands|
|Mexico have failed to win any of their last seven knockout matches at the World Cup||Arjen Robben has won five penalties in 14 Champions League and World Cup games this season|
"It was a nice moment for me but also for the team," said match-winner Huntelaar.
"I had a good feeling and Arjen came to me and asked me do you want to shoot so I took the ball and shot it in the corner."
Netherlands defender Ron Vlaar praised Van Gaal, the Manchester United manager in waiting, for his tactical changes.
"He knows exactly what's going to happen, if it's not happening then he changes to something that works," said the Aston Villa player.
"It gives a lot of confidence to the players - we have a manager capable of changing something and that's great."
Mexico coach Miguel Herrera, who had acquired cult status with his animated celebrations, believes Portuguese referee Pedro Proenca made a mistake with the penalty decision.
"The determining factor was the man with the whistle. He put us of the World Cup," he said.
"Although the first goal was down to our mistake, we had the chance to reconsider during the break before extra time, but at the end when the referee invents a penalty you go out of the World Cup.
"At the very least they can look at this and this referee ought to be going home like us."
Herrera also accused Robben of diving.
"He did it three times," he said. "We'd done really well but then we sit back and we start to give chances to a team that hadn't done anything."
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