Teuns won the ultra-tough mountain stage to Planche des Belles Filles when he and Ciccone crossed the summit finish line as the sole survivors of a mass breakaway.
"It was so steep, but I love these kind of finishes," said Teuns.
"It was a good big breakaway group," said Teuns. "That helped keep us out ahead, five of us were Belgians too."
"When it came down to the two of us, we stayed calm, we talked about it," said the winner, hinting the pair did a deal for a win-win instead of risking it all by not collaborating.
Ciccone was less calm about his feat.
"It's just unbelievable," said the Italian after taking the overall lead from Frenchman Julian Alaphilippe by just six seconds.
Alaphilippe put in a heroic defence of his lead, but missed out due to the bonus seconds over the final two summits saw the young Italian edge ahead of him.
"It feels strange, to have the yellow jersey on my back feels so strange." said Ciccone.
"I wanted to win the stage for the team but I never believed it would be possible to take the lead," said the 24-year-old who rides for the same Trek-Segafredo team as Richie Porte.
The stage ended up as a battle not only for the stage and the overall lead but for the psychological control of the race.
This was the 2019 Tour's first foray into the mountains a stage that the defending champion, Welshman Geraint Thomas, had described as 'the big day'.
Chris Froome won the first ever finish here in 2012 to confirm his potential before he went on to win the 2013 Tour, and Thomas must have sensed something special.
Just behind the breakaway pair, Thomas gave food for thought to those who had called him a one-hit wonder after his 2018 win, who raised eyebrows when he lost a few seconds on a climb at Epernay earlier in the week and who had tipped Ineos team-mate Egan Bernal.
"I don't listen to tips," said Thomas. "I felt pretty good. I thought it would be more of a solid day."
"I was feeling good but I was unsure. I thought the steep climbs weren't my cup of tea."
Thomas and Bernal, who dropped nine seconds to the Welshman, were part of an impressive Ineos team effort that controlled the pace over the most of the course as the peloton crossed seven mountains.
Thomas waited until first Mikel Landa, then Thibaut Pinot and, finally, the overnight leader Alaphilippe himself had attacked before showing his form was back after a nasty fall in the Tour de Suisse in June.
"It's one of those climbs where you really have to be patient," the 33-year-old Thomas said after the dust had settled.
Other good performances on the day came from Briton Adam Yates, Irishman Dan Martin and Frenchman Thibaut Pinot, who grew up in the region.
Three of the overall contenders lost significant time as 2014 champion Vincenzo Nibali, Jumbo Visma captain Steven Kruijswijk and, to a greater extent, Frenchman Romain Bardet suffered on the slopes.
Results from stage 6 of the Tour de France on Thursday a 160.5 run from Mulhouse to La Planche des Belles Filles:
1. Dylan Teuns (BEL/BAH) 4hrs 29min 03sec.
2. Giulio Ciccone (ITA/TRE) at 11sec.
3. Xandro Meurisse (BEL/WGG) 1:05.
4. Geraint Thomas (GBR/INE) 1:44.
5. Thibaut Pinot (FRA/FDJ) 1:46.
6. Julian Alaphilippe (FRA/DEC) 1:46.
7. Nairo Quintana (COL/MOV) 1:51.
8. Emanuel Buchmann (GER/BOR) 1:51.
9. Jakob Fuglsang (DEN/AST) 1:53.
10. Mikel Landa (ESP/MOV) 1:53.
11. Richie Porte (AUS/TRE) 1:53.
12. Egan Bernal (COL/INE) 1:53.
13. Adam Yates (GBR/MIT) 1:58.
14. Daniel Martin (IRL/UAE) 1:58.
15. Rigoberto Uran (COL/EF1) 2:02.
16. Michael Woods (CAN/EF1) 2:02.
17. George Bennett (NZL/JUM) 2:02.
18. Enric Mas (ESP/DEC) 2:17.
19. Bauke Mollema (NED/TRE) 2:19.
20. Steven Kruijswijk (NED/JUM) 2:19.
21. Warren Barguil (FRA/ARK) 2:27.
22. Vincenzo Nibali (ITA/BAH) 2:35.
25. Fabio Aru (ITA/UAE) 2:46.
27. Romain Bardet (FRA/ALM) 2:53.
29. Alejandro Valverde (ESP/MOV) 3:05.
31. Roman Kreuziger (CZE/DDT) 3:26.
36. Thomas De Gendt (BEL/LOT) 6:36.
47. Simon Yates (GBR/MIT) 11:12.
59. Wout van Aert (BEL/JUM) 14:46.
66. Greg Van Avermaet (BEL/CCC) 14:51.