I have searched for this already and I did not find it.
This is very much the same as the hunter pet shizzle, when u run away from it and then re summon it, the pet gets 100% hp.
Well with DK's, all you need to do (if ur ghoul is on low hp, lets say 20%) is mount up (pet disapperars) and then get off ur mount (pet resummons with 100% health).
I have only tried this with Unholy DK as the ghoul is like a hunterpet with a spec (Master of Ghouls).
Thanks!