That it was a bug/exploit and unintended, there were a couple of workarounds to overclock the HBM on Fiji but it was supposed to be disabled in the driver.Now if AMD Matt states HBM overclocking was not enabled in driver and I showed you a screen shot of HBM clock slider in AMD drivers prior to WattMan what would you say?
One of the cheats was to manually edit an Afterburner profile and then it would apply the clocks in that profile (even though you couldn't set them normally). Also sometimes every few reboots the slider would magically appear in the drivers (on some versions) even though it wan't supposed too, this allowed the HBM to be overclocked until reboot.