England were a stand-out in the first round of pool games, trouncing Argentina 27-10 in Marseille despite playing all but three minutes with just 14 men because of Tom Curry's red card.
It was a marked turnaround from their World Cup warm-up game at Twickenham when they suffered their first-ever loss to Fiji, which followed defeats to Wales and world No.1 Ireland.
England's head of strength and conditioning Aled Walters revealed the team was in heavy training leading into those matches, following a lead set by Sweetenham.
The 73-year-old Queenslander managed the Australian swim team for four Olympic Games and five Commonwealth Games before joining Great Britain as the national performance director from 2000 and overseeing their campaign at the 2004 Games in Athens.
Walters said four days before taking on Ireland Steve Borthwick's side played a full contact game behind closed doors.
"Bill Sweetenham is a very famous Australian swimming coach who worked with the British Olympic team years ago and he made sure the swimmers went into the world championships, which just preceded the Olympics, tired," Walters said.
"They got some medals but he knew if you can perform when you are tired and lacking energy, what are you going to be like when free of fatigue and energetic. That is what we are getting to now."
Walters, who joined the program in June and was part of South Africa's 2019 title-run, said it was "amusing" to see the reaction in England to their lacklustre showing, knowing what was ahead when they tapered.
"People were thinking, 'There's a World Cup round the corner and they look terrible, they don't look fit, they're lacking energy'," Walters said.
"But we knew exactly what was coming and the players did as well ... to get someone fitter you almost have to dig them into a hole first to allow them to recover then come out stronger.
"Yes, we lost (to Ireland) but we went to one of the hardest places to go and it served as preparation going into Argentina and now Japan.
"We had confidence in our planning and our program and that short-term pain is paying off now as the fresher they get the stronger they'll be."