An interview with Chad Haga (Instagram: @hagasaki) on the Cycling Tips podcast revealed that he and Victor Campenaerts (Instagram: @campenaertsvictor) had been quietly marking each other closely through the mountains, while doing as little work, and saving as much energy as possible in preparation for the stage 21 individual time trial in Verona.
Here's the time split swarm chart highlighting their progression down the GC rankings as part of their preparation. The vertical axis shows the number of seconds a rider is behind the leader of the general classification competition. Per the colours of the teams they were racing for at the time, orange is Haga, red is Campenaerts.
The end result: Haga and Campenaerts were first and second respectively on the day, with Thomas de Gendt rounding out the podium. See the full results from the day here.
PS: I was pretty stoked to pick up a comment from Victor in the comments on Instagram. :-)