The 1815 Tourbillon is not really talked about much these days, at least not in the way designs like the Datograph or Zeitwerk are. But it remains a horologically impressive achievement from the brand, as the world’s first zero-reset and hacking tourbillon upon its launch in 2014. Though we all take hacking for granted on most watches these days (and often scoff at watches that don’t hack), hacking a tourbillon is a whole different endeavour. Yes, in concept, all hacking seconds are largely the same, with some sort of braking force applied to the balance wheel to halt it. But with the tourbillon, you have the added physical obstacle of a tourbillon cage surrounding the balance wheel, not to mention the added mass that needs to be halted.
With the first 1815 Tourbillon in 2014, Lange devised a V-shaped spring, designed to make contact with the balance wheel once the crown is pulled out and stop the mechanism in place. When the crown is pushed back in, a lever pivots the arm holding the spring away from the balance wheel, and the balance starts oscillating again.