The “tactic,” subtle as it was, was to not completely burn out Dennis. The usual Sky MO in the past was for Porte/Poels/Landa to drill it until Froome could attack and drop the rest, but in this case they knew their guy wasn’t stronger than Sunweb’s last guy, so having Dennis go flat out would have played into Hindley’s hands.Not sure what race did you saw. Dennis didn't cover any of the Hindley attacks, it was Tao. Dennis recovered because Hindley's attacks were too short and weak to drop Dennis.
Tactics? If Hindley would've been much more powerful than Tao, Ineos's tactics would've been shambles.
After Hindley’s first attack, Dennis came rolling through right back to the front almost before Hindley sat up. Tao covered it, but Dennis made sure Hindley knew he was still there. It meant Hindley knew that whatever attack he did put in, Hart would not be chasing him alone, but would have support.
So even if Hindley had been a lot stronger than Tao (and the indications from the early going were that he was) Dennis was still available to help Hart. Also, by Dennis not going fill gas once the gap to Kelderman was established, it meant Hart was (relatively) fresh, and able to respond to Hindley’s attacks. Again, if Dennis went 100% on the climb, chances are he would have dropped Tao, or at least left him vulnerable to attack.
It’s the first time Ineos have had to deal with another team’s leader being stronger than theirs. It does help that their no1 domestique is stronger again, but they’ve also had to make adjustments to their usual plan, and they’ve maximized their guy’s chances of winning the race.
They’ve been very smart the last 2 mountain stages. On the last rest day, I think pretty much nobody was giving Tao a chance of winning the Giro, yet here we are with him tied for 1st going into the final TT.