Keith Curle pinpointed the timing of Notts County's opening goal as a crucial factor after they ran out 2-0 winners at Fratton Park.
Francois Zoko's headed the Magpies ahead in first-half stoppage time, before Yoann Arquin nodded a second after 56 minutes to clinch victory for the visitors in their FA Cup first-round tie.
The Notts County boss was delighted to see his side take charge at an ideal time, moments before the break, and said: "We knew we had to stop Portsmouth from playing their game and disrupt their rhythm, while also being an attacking threat ourselves.
"Goals change games. When we scored just before half time, it slightly deflated them.
"But the important thing for ourselves was how we started the second half. We didn't just sit back and defend that lead, there were goals for us to have.
"The second one gave us a comfort zone and it meant we could play to our shape and not force the game."