Jose Mourinho said he gave his Chelsea players a half-time ultimatum to inspire them to a 2-1 win over Porto.
Mourinho admitted Porto deserved their half-time lead
Porto were 1-0 ahead at half-time but second-half strikes from Arjen Robben and Michael Ballack put Chelsea into the Champions League quarter-finals.
"I had a difficult half-time because at that point, we were out of the competition," said the Chelsea boss.
"I told them either we respond to the situation or we are scared of it. If we are scared of it, it is goodbye."
He added: "Psychologically, I just made the players think a little bit.
"Porto did better than us in the first half. Not because they were winning 1-0 but because of the way they were playing.
"From our very strong second half, we deserved to go through."
Mourinho reserved special praise for Ballack, who has often struggled in his first season with Chelsea.
"I think the goal is good for him because it put Chelsea in the quarter-finals," he said.
"It is one of those goals that is like a stamp. He put in a great effort to play a good game. He was exactly like the team - didn't play well in the first half, but played very well in the second half."
Mourinho believed the introduction of Mikel Jon Obi at half-time made Chelsea more "dynamic" in midfield and Porto boss Manuel Jesualdo Ferreira agreed the substitution might have been a turning point.
"We were playing well in the first half, but then came a change by Chelsea," he said.