The center button was meant to bring up a sub-menu initially, which gave you extra functions. But those extra functions were so unstable that Hifiman decided to disable them right after the first firmware reversion until they can sort them out properly. So at the meantime, it is just another play / pause button.
Hey ClieOS, word behind the scenes is you may be retiring from doing reviews for awhile. I think I speak for the majority on here in saying we all hope the rumor is false. I'll admit I haven't always seen eye to eye concerning some of your reviews but overall the vast majority of what I've read from you have been quite entertaining and spot on. It would be a loss to the community it you decided to retire.