Hi there,
Found this out randomly on my hunter during a heroic raid, during a failed attempt I used feign death to avoid dying.
The thing is, this was during the boss casting infesting spores which is a raidwide stacking debuff. When feign death triggered all the stacks fell off me and started to restack from 1. This means you could drop them around half way through infesting spores to minimize damage on you greatly. This helps a lot when you're doing the flamethrower as you may be out of range of the healers. (lets face it, as a hunter you're always on the flamethrower)
Feign death shouldnt give you any immunities but im not 100% sure if this may be working as intended. Either way a neat trick.
Hope this helps!