Steve Cotterill admitted he had to bide his time to make his tactical switch in Saturday's win at Norwich City.
The Blues had done some intensive work on the training ground in the run-up to the trip to Carrow Road in a bid to combat City's diamond midfield formation.
But despite success on the training pitch with everyone knowing their roles, it failed to pay off in the first half as the Canaries threatened to run riot. And Cotterill had to trust in his men to get to the interval before making the switch to a 4-5-1/4-3-3 formation that transformed Pompey for the second period.
Cotterill explained: 'It wasn't working for us and about 10 minutes before half-time, I was going to change it but I needed to wait to explain it properly in the dressing room.
'When teams play the diamond formation it's difficult.
'I felt we dealt with it in certain quarters of midfield in the first half, but when one of the quarters doesn't quite deal with it, it gives you a problem.
'We worked through the week on how we were going to do it and it worked well. Everyone understood it.
'There was one side of the diamond who was causing us a problem and we didn't adjust to it as we had been adjusting to it in training all week.'
The change to push David Nugent out wide on the right flank and bring Liam Lawrence inside into a midfield three gave Pompey a new dimension going forward after the break.
It proved Pompey can be flexible in their style of play, despite their limited squad numbers.
Cotterill explained: 'What is difficult when you play against the diamond is they can monopolise the ball and their full-backs can run you back.
'But when they do that, they will leave spaces in behind and you need to get them turning.
'We felt we may cause them a bit of a problem if we could get our front three higher up the pitch in the second half.
'Thankfully, we caused them a couple of problems and it worked much better.'