Originally posted by: varaali
While Shantanu may have been a great ruler et all, he was no great rishi to have the power to give such a boon.
This is how I understand the situation - What Shantanu gave Bhishma was an emotional filled paternal blessing. Like our elders say " Chiranjeevi Bhava". Similarly Shantanu may have blessed his son that may death never touch him. But there was no guarantee that he blessing would actually fructify.
So how did Bhishma then manage to hold off death till he wished? The answer probably lies in his practice of brahmacharya (celibacy). Brahmacharya does not only mean non- marriage but total restraint over mind / body and body functions. I am afraid I cannot be more explicit than this but, you can understand what I am trying to imply
Our Yoga - Shastras unequivocally claim that there is a direct co-relation b/w the practice of brahmacharya and longevity. Moreover, practice of brahmacharya is supposed to help in the practice of other yogic techniques.
In fact the death of Bhishma is beautifully described. In his final few moments, Bhishma closes all the nine openings of the physical body and directs his prana slowly upwards until it breaches the crown / sahsrakara chakra and through that aperture Bhishma's prana leaves the body.
So coming back to the point- what Shantanu gave Bhishma was just a paternal blessing and Bhishma was able to make it come true through his own practice of brahmacharya / yoga.
This is my understanding.