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I believe the bidirectional functionality was to make it easier to time and reset navigational waypoints while underwater. That’s also why the original FXD has a countdown bezel.
I fully understand the purpose of a unidirectional count-up bezel on a divers watch, I just didn’t understand why they had changed to that more conventional design on this new version, when that was part of the FXD’s raison d’etre. But then I read the marketing guff about US military spec divers watches and it makes sense.
The FXD, when it first came out, was supposedly designed to the MN’s specs, which included a fully minute-marked bidirectional bezel. They didn’t want unidirectional. So all the FXDs up to this point have been bidirectional (and therefore not an ISO-compliant divers watch).
Seems this one is meant to comply with US army specs, hence unidirectional.