To perform this bug you need Zenk'aram, Iridi's Anadem (legendary head) and be in M+ alike environment, where you constantly go in and out of combat. That being said, it is game breaking bug in M+ environment. It works since 7.2 release (head was buffed).

This is how random scenario with this bug looks like: you're in combat killing some trash mobs with voidform, after combat ends you swap asap to legendary head and swap back to "proper" legendaries. Equippin that legendary head resets insanity drain while voidform stacks are still high like 50-100. Then you simply pull another pack of mobs, kill it, swap to legendary head, swap to proper legendaries, repeat.

Properly executed you can run entire instances doing so, each time pulling 3-4mln DPS on packs.

It was abused for example by Viklund on his stream and it is abused by other top shadow priests.

It should be fixed, and it definitely should be fixed before ToS, otherwise it will have both direct and indirect impact on progress race.