Saturday saw Spurs end their 17-year wait for a win at the home of their arch-rivals when they came from 2-0 down at half-time to triumph 3-2 at the Emirates Stadium.
The introduction of Defoe during the break was the catalyst for the comeback, with the England striker's flick-on helping set up Bale to make it 2-1.
Rafael van der Vaart's penalty and Younes Kaboul's late header then sealed one of the most improbable and memorable derby victories ever recorded by Tottenham as Arsenal blew the chance to go top of the Barclays Premier League.
Defoe said: "Second half, I think they eased off a little bit.
"Sometimes, it's difficult when you go 2-0 up. People think you just go on and you win the game quite comfortably.
"But sometimes you take your foot off the gas.
"I think they did that and when we got the goal back – the first goal – after that I knew we'd win the game."
The victory, which closed Harry Redknapp's men to within four points of Arsenal and six of leaders Chelsea, was Spurs' first in 69 games at the home of one of the so-called 'big four'.
Defoe said: "As players, you don't dwell on the past and what's happened before.
"It's a new day and you always look forward to the games.
"We had a fantastic season last year; we want to try to do the same, just keep getting points and try to get in that top four again."
Defoe's 45-minute run-out was his first since recovering from the ankle surgery that had sidelined him for two months.
"It's amazing really," said the 28-year-old, who replaced Aaron Lennon as Tottenham went for broke.
"Aaron said after the game, 'Were you tired?', and I said, 'Not at all'.
"I put all the work in on the rehab stuff, so I knew I'd be
"The ankle feels good, which is always nice, and it was a great result."