South Africa beat England 32-12 to join New Zealand as the only sides to have won the trophy three times each.
Du Toit spoke at the launch of a documentary outlining the path to World Cup success for the side. It covers the period from their 57-0 loss to the All Blacks in Albany in 2017 through to their Cup triumph.
Du Toit said: "For me, when I realised that 'we've got them', they were behind their tryline and started running from there, and threw a 20 or 30-metre pass…I thought, 'Listen, are you guys crazy? This is a World Cup final, and you don't play like this'."
"The other one was when [Billy] Vunipola picked up at the back of the scrum and ran into [five-eighths] Damian de Allende and Handré Pollard, and he just threw the ball away and no one caught the ball.
"It just showed that we were quite physical in that game and that they were quite afraid of us. That's what the plan was," he said.
Prop Tendai Mtawarira, now retired, said when South Africa lost their opening round game to the All Blacks it had made them wonder whether they could go all the way to a third world title. However, coach Rassie Erasmus had been able to convince them they were good enough.
"After we lost to New Zealand, Rassie told us that 'we are still going to win this World Cup'. Despite not starting on the right step, he put that belief in us. Sitting in that change room, most of the guys had doubts," he said.
"But having your coach tell you that you are going to win this World Cup – even though we lost – was so big for the team.
"He told us that 'we only have four big games left to hold the trophy: Italy, quarterfinal, semifinal and final.
"Our whole mentality shifted, and we just started to believe," he said.