Louis van Gaal says Manchester United cannot play 4-4-2 because it gives him "a twitchy ass".
United boss Van Gaal has come under fire for persisting with three at the back, a system many of their fans and ex-players feel inhibits the former champions.
But ahead of United's FA Cup trip to League Two Cambridge United, Van Gaal revealed why he is sticking with his much-maligned system.
Former United manager Sir Alex Ferguson coined the phrase "squeaky bum time" and now Van Gaal has delivered his own memorable saying.
"I know the stats are giving the positive results that we have won more with 4-4-2 in a diamond," he said. "But when you analyse the game then we have twitched our ass on the bench.
"I said to my players I was 'squeezing my ass', but it was the wrong expression. I have twitched my ass on the bench because we were out of balance.
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"We lost the game against Leicester City 5-3 and that was undeserved. And all the matches when we play with 4-4-2, like against West Ham, it was always twitching your ass and I don't like it.
"Maybe then you see, after I switch the system, we have won nine or ten in a row.
"Then we lose one match and then the discussion about the system is coming, so it's ridiculous. In the last 15 games we only lost one [to Southampton] and that was undeserved - we were the better team."
Van Gaal is expected to stick with 3-5-2 for the visit to Cambridge, despite being without injured defenders Jonny Evans and Chris Smalling, as well as wing-back Ashley Young.
United legend Gary Neville claimed his former club had become "predictable" playing 3-5-2 and that their tempo was "slow" under that system.
But Van Gaal said he had no time for critics of his approach and insisted he would not be swayed into changing his style by fans or pundits.
"I have to communicate with my players, observe my players, analyse my players, my team, my opponent and the quality of my opponent, then I have to make a decision with my staff," said Van Gaal.
"But I cannot hear or listen to what the media are writing or what other people are saying or writing, because they are never at the training ground and not attending the meetings.
"My staff are attending the meetings and my players, so I have to work with them, and I have a board that believes in the manager and these players."
Van Gaal admitted United's players were still not comfortable playing three at the back, but said they were becoming more familiar with the system and what is required of them.
"They are still not comfortable," said Van Gaal. "That is very important that they are comfortable.
"I would like them to feel more comfortable. But we have to do our preparation in the season so we are doing a lot of things during the matches.
"Of course, it's very important to win all your games, but that's not possible in the Premier League.
"But we've had a fantastic run of results except Southampton at home.
"Up to now I'm pleased and we're going in the right way. But we have to beat Cambridge United and that is Gladioli or the death [glory or failure]."
Van Gaal also defended his controversial decision to play record £59.7million signing Angel Di Maria as a striker in United's past two games, rather than in his more familiar wide role.
"He played in the 4-4-2 in the left midfield position," said Van Gaal of the Argentinian. "In the 3-5-2 he hasn't played so much because he's been injured.
"He came back and I tried him as a striker, because we need pace in our striker's position and we have to stretch the pitch. That's also an important aspect of a striker, that he can stretch the pitch.
"Then you make the pitch bigger and your midfielders can play in a bigger space and play a better positional game. So I tried that because I didn't have the preparation time [with Di Maria] at the beginning of the season."