It looks an awful lot like the mode dial on a current Pentax, which would mean you hold the button down with a finger to release the lock on outer dial and then turn the dial with your thumb.
Is it going to be a toggle, up to change the shutter speed and down to change the ISO? If not and the button locks the ISO that means no lock for the shutter speed. If the button is only for the ISO, I hop it is either a toggle or just a lock for the A position.
I think the outer, ribbed rim of the dial is a separate piece from the top cover with the markings. I believe that's how Spotmatic worked.
To change the shutter speed you just turn the dial like you would normally do it. There appears to be a lock, but it's still unknown if it's a toggle, like on Olympus cameras, or a regular lock as on the X-pro 1.
To change the ISO value, you lift the outer rim of the dial and you turn it. I believe the H A L stands for "high", "auto", and "low", just like on the X-T1. That's how Soptmatic worked I think.
So far I think there's one positive with this dial : Fuji's modified the way the body wraps around it. They've made more space for your front or rear fingers to move across the body. The X-pro 1's dial was badly designed in that regard.
The negatives with such a solution are many :
1) If the lock is like the older X-pro 1, it's just the sort of thing that I totally hate to use. Because of how slow it is to access the minimum shutter speed in auto ISO on most cameras, and how slow it is to change custom settings, and particularly the Fujis, I'll very frequently want to go from A to manual shutter speed. Since there's a lock, I'll always need to loose my grip on the camera to do this. And anyway, a properly designed dial doesn't even need a lock.
2) It's even worse for the ISO dial : since the dial has to be lifted every time you want to change the ISO value, you'll be constantly loosing your grip every time you want to change that value. It's like the DF's PASM dial.
3) For both operations, you'll need to use at least two fingers, except for changing from a manual shutter speed value to another one, presumably. It's going to be a PITA when looking through the VF for example.
4) The ISO dial doesn't have three "A" positions, to match Fuji's not so dumb idea of being able to register three different auto ISO settings, as on the X100T.
5) When the shutter speed dial is set to certain values, you'll be reading the ISO values upside down.
EDIT : I've read another suggestion, that the lock can be used to "uncouple" the outer rim from the top cover. That would make point 1) and 3) a lot less problematic to me.
