When one washes Netilat Yadayim upon waking in the morning, he should use enough water that it covers both sides of each hand, until the wrist. One should move his hand somewhat as he pours the water on it, to ensure that the water covers both sides, the back and the palm.
If one has a limited amount of water for Netilat Yadayim, which does not suffice to cover the entire hand, then he should wash the back of the fingers, until the knuckles where the fingers meet the wrist. When one cannot wash the entire hand, this minimum amount suffices to fulfill the Misva, and he recites the Beracha as usual. The Ish Masliah (Rav Masliah Mazuz, Tunisia, 1912–1971) writes that one may – and in fact should – rely on this leniency even if he can go to a different house to bring more water. It is preferable to wash one's hands promptly in the morning with the minimum amount of water than to delay the washing until one goes elsewhere to bring the preferred amount.
On Yom Kippur and Tisha B'Ab, when washing is forbidden, one washes his hands with the minimum required amount, washing only the back of the hand, until the knuckles. The Kabbalists explain that although the forces of impurity generally cling to the entirety of the hand, on these two days – Yom Kippur and Tisha B'Ab – they cling only to the fingers. On Yom Kippur, due to the high level of sanctity that a person attains, the forces of impurity are incapable of extending beyond the fingers. And on Tisha B'Ab, the forces of impurity feel overconfident in their ability to cause harm that they are content remaining on the fingers. This explanation is brought by the Kaf Ha'haim (Rav Yaakob Haim Sofer, Baghdad- Jerusalem, 1870-1939) and the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909).
If one has a limited amount of water for Netilat Yadayim before a meal, and must wash only until his knuckles, then, according to the Ben Ish Hai, he does not recite the Beracha of "Al Netilat Yadayim" over this washing. Hacham Ovadia Yosef, however, disagreed, and maintained that the Beracha may be recited in such a case, since the person satisfies the obligation, albeit on the minimum standard.