1) HALACHAH: THE TIME FOR BITUL
QUESTION: The Gemara explains that Bitul Chametz is performed at the time of Bedikah, because at that time one is involved with getting rid of his Chametz and thus he will remember to be Mevatel it.
Bitul cannot be done during the sixth hour of the day on the fourteenth of Nisan, because the Chametz at that time is already forbidden. Bitul is not done before the sixth hour, when one is still permitted to derive benefit from (but not to eat) the Chametz, because of the concern that one might forget to perform the Bitul since no designated act or moment is associated with it.
Is this the Halachah in practice today?
ANSWER: The ROSH (1:9) cites a Teshuvah of RASHI in which he writes that nowadays one should do Bitul not only at the time of Bedikah but also at the end of the fifth hour on the fourteenth of Nisan. The end of the fifth hour indeed is a time "designated" by an act which will remind one to perform the Bitul, because it is at this time that the Chametz is burned (immediately before it becomes forbidden entirely). This second act of Bitul is done in order to be Mevatel any Chametz that might have remained from the bread that one left over for the morning of the fourteenth, which he has not yet annulled. This is the ruling of the SHULCHAN ARUCH (OC 434:3).
2) A BLESSING FOR A MITZVAH "OVER LA'ASIYASAH"
QUESTION: The Gemara says that one recites a blessing for a Mitzvah "Over la'Asiyasah" -- before he performs the Mitzvah. Why does the Gemara use this unusual phrase and not say simply "Lifnei Asiyasah"?
(a) The NIMUKEI YOSEF (Hilchos Tefilin, DH Over) explains that "Over la'Asiyasah" means "ahead of" the Mitzvah that will be performed, and not "before" the Mitzvah will be performed. This means that one must first begin to become involved in the performance of the Mitzvah (such as by lifting the Shofar or Lulav, or by beginning to wrap the Tefilin around one's arm), and then one "runs ahead" of the Mitzvah and recites the blessing immediately before the actual fulfillment of the Mitzvah.
This definition of "Over" is evident from the verse that the Gemara cites as proof for the meaning of "Over": "And Achima'atz ran... and overtook (va'Ya'avor) the Kushi." The verse says that the Kushi was ahead of Achima'atz, but Achima'atz overtook him and went before him. The second and third verses cited by the Gemara use the word "Over" in a similar manner. Yakov first lined up his family before him, and then he passed before them. Similarly, after the nation lines up, the king passes before them to lead them (and he does not wait in front for the formation to form behind him). (See also Insights to Nidah 63:2.)
(b) The SHA'AGAS ARYEH (#26) explains that the word "Over" can mean either before or after (in the past tense, "Avar"). The Gemara uses the unusual phraseology of "Over la'Asiyasah" in order to teach that, b'Di'eved, one may recite the blessing on a Mitzvah even after the Mitzvah has been performed, as the HAGAHOS ASHIRI (Berachos 1:13) rules (in contrast to the ruling of the RAMBAM in Hilchos Berachos 11:5).
(c) One of the Gemara's sources that the word "Over" means "before" is the verse, "Their king passed (Over) before them, and Hash-m was at their head." Perhaps the Gemara's use of the word "Over" to describe the way a blessing must be recited alludes to another principle in the laws of blessings: every blessing must begin with a mention of the name of Hash-m and His kingship, as the Gemara teaches in Berachos (49a). This is expressed by the word "Over," which is used together with a mention of the name of Hash-m in the verse, "and Hash-m was at their head." (M. KORNFELD)