Backdraft only reduces the GCD to 1 second (hard cap) and not below it.
Someone didn't study their patchnotes I see:
From the 3.0.8 patchnotes.
* Backdraft now also reduces the global cooldown of your next three Destruction spells by 10/20/30%.
The current ingame tooltip
When you cast Conflagrate, the cast time and global cooldown of your next three Destruction spells is reduced by 30%. Lasts 15 sec.
The old ingame tooltip
When you cast Conflagrate, the cast time of your next three Destruction spells is reduced by 30%. Lasts 15 sec.
In other words, it used to just give you 30% reduction, applied in the same way as bloodlust (ie, multiply by 1/1.30) . If your GCD without backdraft was 1.2 sec, it used to just multiply that, giving you a 0.92 sec cast on immolate, but leaving the GCD at the 1 sec hard cap.
Now, it actually reduced the GCD too, meaning your GCD can go below the usual hard cap. That is to say, a new minimum GCD of 1/1.30 = 0.77 seconds with backdraft.
Now, for backdraft you simply multiply your normal hasted GCD (still with the 1 sec "hard" cap) by ~0.77.
So, if you have excactly 50% haste = 1 sec base GCD, your backdraft GCD will be 0.77 sec. If you have over 50% haste, your base GCD will still remain 1 second (though your immolate cast time will be shorter), and your backdrafted GCD still 0.77 second (and your backdrafted immolate cast time even shorter).
Kinda hard to explain, but hope you get the gist of it.
If you've played a destro lock with a decent amount of haste (20% or so) pre-3.0.8, you would have noticed this when under bloodlust. This would give you a base cast of 1.19 seconds on Immo, giving you a backdrafted cast of 0.92 seconds, meaning for 0.08 seconds after an immolate, you are under the GCD. When chaining spells (by this i mean, if you try to cast a new spell in the last ~10% of the cast of the previous spell, it will auto-cast the new spell after the previous finishes), you would get a "this spell is not ready yet" error, kinda messing up your flow, and underutilizing your haste. And due to the new glyph of conflag, you will rarely cast immolate backdrafted, diminishing the problem with bloodlust+20% haste (a base cast that's lower than base GCD).
The consequence of this new way backdraft affects the GCD, is obviously that deep destro locks will enjoy haste a lot more. Not only has it always scaled better than crit when fully raidbuffed (ie, ~20% base+5% devastation+5% boomkin+3% wrath+10% imp scorch+8% 2-piece t7 setbonus average+7% Empowered Imp average ~= 58% crit average), but it no longer has that annoying "clipping" when using backdrafted immolate.