Boonen spoke with Sky's Luke Rowe ahead of the key moment. The winning split left a group of 14, with Boonen and team-mates Matteo Trentin and eventual winner Philippe Gilbert, and Sky's Rowe and Gianni Moscon.
"I was expecting Sky to go full-gas on the Muur," Boonen said.
"There was another race, I cannot remember when, but they tried some old cyclocross trick, when you have many guys go hard to the base, then pedal at three kilometres an hour, and then everyone behind you has to step out of the pedals, and then those at the front accelerate again.
"But then Rowe asked me if we were going to attack, and then I realised they were more afraid of us. So I said, 'screw it,' and decided to go for it."
Quick-Step, Sky and Direct Energie – with Sylvain Chavanel and Bryan Coquard – held the upper hand. Quick-Step, with three, let Gilbert keep his advantage after the Oude Kwaremont, which turned into a race-winning 55.5km solo ride.
"We ended up with a good group there, and everyone wanted to work. That was one of the nice things over the past few weeks, and everyone was only talking about two riders. We all wanted to ride together," Boonen continued.
"When we got close to the Kwaremont, I asked Phil how he was feeling, and said really good. I said, 'OK, I will go all-out to the base of the Kwaremont to try to block the race behind.'"
Rowe remained with Moscon, but crashed into a falling Sep Vanmarcke (Cannondale-Drapac) after the Oude Kwaremont with 52km remaining.
Boonen rode defensively behind and readied for the counter-moves. Given the chance, he could have raced for a fourth and record Tour of Flanders win. That changed quickly when Boonen suffered from bike problems on the Taaienberg at 37km to race.
"I was changing from the big ring to the small ring, and my chain slipped, and got stuck between the frame," added Boonen. "I pulled on it once, but it was really jammed. Then I got on another bike, and the same thing happened.
"They gave me Niki Terpstra's bike, and it was too small for me, and I knew then my race was over. Mentally that was very hard, because the race was going well."
Boonen, 36, ends his career with two more races. He will take part in the Scheldeprijs, which the organiser moved to start in his hometown of Mol on Wednesday, and Paris-Roubaix on Sunday.
He already won Paris-Roubaix four times, a fifth would set a record.
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