Finally got to try this new Ninjutsu out and i´m impressed ^^b
Tanked Fafnir as NIN/DNC in a 5 man party last night (was NOT expected to do that o_o' ) and after around 10% of it´s HP down it did "Dragon Breath" which hit me for 1500+ dmg, fell to the ground instantly since i didn´t have time to throw on fire resist gear...
Luckily the mages kept it cool and managed to hold it while i Mijin Gakue it and got another raise.
This time i threw on this spell and next Dragon Breath... 0 dmg! The one after that... 0!
when i started to throw on my Fire resist gear while Miguwari was still on i started to take dmg again, everything from 700 to 1398 (2 HP left...) which makes me believe this spell only saves you from dying. If a mobs TP move (or magic?) is devastating enough to kill you this jutsu kicks in and absorbs the damage, if it would take 90% of your HP but not kill you it will not kick in.
Every time the ninjutsu did kick in it disappeared so you´d have to wait for cooldown or recast it (definately recomend throwin on hastegear while casting it, that way it can be up 2/3 of the fight).
Hope this helps ^^b