When we studied Tractate Yadayyim we learned that the minimum amount of water that may be poured over the hands to effectively perform netilat yadayyim is one revi'it. We also noted that this word, which means 'quarter' refers to one quarter of a log of water. Now, one quarter of a log of water is a rather small amount - around 120 cc's - which is less than a usual glass of water. Paragraph 10 of Section 158 seeks to emphasize the fact that one revi'it of water is the halakhic minimum; presumably it is to be permitted only when water is in very short supply: under normal circumstances one should perform netilat yadayyim using a generous amount of water.
The idea that a generous amount of water should be used for netilat yadayyim is prompted by a saying attributed to Rav Ĥisda. (Rav Ĥisda was a prominent Babylonian amora [Talmudic sage] who died in the year 309 CE.) Apparently, Rav Ĥisda was wont to say that he would be generously rewarded by Heaven for washing his hands in generous amounts of water, even though the strict minimum was just one revi'it. (The saying is reported in the Gemara: Shabbat 62b.) Nowadays it is customary to use a special natla from which to pour the water over the hands, a utensil which usually contains a generous amount of water.
Mitzvot which involve the performance of a required action must be preceded by their special benediction. There are numerous examples: we recite the benediction "to listen to the sound of the shofar" before sounding the shofar on Rosh ha-Shanah; we recite the benediction "to take the lulav before shaking the 'four species' on the festival of Sukkot. In order that there be a clear connection between the benediction and the required action it must immediately precede the appropriate action with the smallest intervening interval as possible. This requirement that the benediction immediately precede the action itself applies not only to mitzvot that derive directly from the Torah: the sages also applied the same requirement to the seven mitzvot that they initiated. (See, for example, the rather convoluted discussion concerning the berakhah upon lighting the Shabbat candles. You can find that discussion in Shabbat 020.)
Logically, one would think that the benediction over washing the hands should be recited before performing the action. (Indeed, as Rabbi Israel Me'ir Kagan points out in his commentary Bi'ur Halakhah, if someone is certain that their hands are clean before washing them they may recite the benediction before they pour the water over their hands.) However, we recall that one of the reasons for instituting the requirement to wash our hands before eating bread was because our hands are 'fidgety': they get into all sorts of unclean places - including our own bodies - without us even being conscious of this fact. It is not appropriate to recite the benediction with impure - and even dirty - hands. Thus it has become the almost universal custom to recite the benediction after pouring the water over our hands.
Thus there are several possible ways in which the benediction which accompanies netilat yadayyim may be recited:
- before pouring the water over the hands (if one is absolutely certain that one's hands are clean);
- after pouring the water over the hands the first time and before doing so for the second time;
- after pouring the water over the hands for the second time and before drying them (and this is now generally considered to be the optimal manner of performing the mitzvah and is almost universally observed);
- after drying the hands in the event that one had forgotten to recite the benediction before drying the hands.
One of the reasons that reciting the benediction before drying the hands is considered "immediately preceding" the performance of the mitzvah is because the drying of the hands after washing them is an integral part of the performance of the mitzvah: paragraph 12 of section 158 points out that if the hands have not been properly dried the mitzvah has not been properly performed. We recall that one of the reasons that hand-washing was instituted was to prevent priests imparting ritual impurity to their terumah foodstuffs because of contact with their ritually impure hands. (See Shabbat 035.) It logically follows then that it someone did not dry their hands properly they have not yet fulfilled the mitzvah, their hands are still ritually impure, and when they touch the bread they pass on to it the ritual impurity.