(a) TOSFOS (12b, DH Eimasai) explains that "She'as Bi'uro," the time of its
destruction, refers to the time *prior to midday* on Erev Pesach, during
which people are involved in getting rid of their Chametz. "She'Lo b'She'as
Bi'uro," or not at the time of its destruction, refers to the time *after*
the prohibition of Chametz takes effect, at the end of the sixth hour on
Erev Pesach, and throughout the festival. During that time, if one finds
Chametz he must burn it according to Rebbi Yehudah, but before that time,
when people are involved in getting rid of their Chametz on Erev Pesach, one
may dispose of his Chametz in any manner.
The logic of Tosfos is simple. Before Pesach, there is no obligation, no
Mitzvas Aseh, requiring one to do something with his Chametz. Rather,
getting rid of his Chametz is merely a "Hechsher Mitzvah," a preparation for
a Mitzvah. One is not fulfilling the actual Mitzvah of Tashbisu. The Mitzvah
of Tashbisu applies only after Pesach starts. It is a qualifying element of
the Isur of Bal Yera'eh. That is, Bal Yera'eh says that one should not have
Chametz in his possession on Pesach, and Tashbisu tells us what must be done
with the Chametz if one does have it in his possession during Pesach. In
effect, Tashbisu makes Bal Yera'eh into a "Lav ha'Nitak le'Aseh," a
prohibition which can be rectified by the performance of a Mitzvas Aseh, for
which Malkus is not administered (Tosfos 29b, end of DH Rav Ashi).
(b) RASHI (12b, DH Rava Amar) explains that "b'She'as Bi'uro" refers to
*after* Chatzos on Erev Pesach, when the prohibition of Chametz has already
taken effect. At that time, one may dispose of Chametz in any manner. Shelo
b'She'as Bi'uro, during which Chametz must be burned, refers to the sixth
hour of the day, i.e., *before* Chatzos, on Erev Pesach. At that time, Rebbi
Yehudah maintains, the Mitzvah is specifically to burn Chametz.
It seems that Rashi understands the Mitzvah of Tashbisu to apply *before*
Pesach. Tashbisu is the Torah's command to prepare for Pesach before Pesach
arrives; it tells us to get rid of Chametz that is in one's possession by
chemically changing its form through burning it, until it is totally
nonexistent. When one burns his Chametz before Pesach he is doing the
Mitzvah d'Oraisa of Tashbisu. After Chatzos, though, it must be destroyed in
any manner possible. Once the sixth hour has passed and the Isurim of Bal
Yera'eh and Bal Yimatzei apply (see Insights to Daf 4:1 and Rashi DH 4a, DH
Bein l'Rebbi Meir), the Torah no longer asks us to get rid of our Chametz in
the best way possible. If Chametz is in one's possession he must get rid of
it in *any way* possible. That is, Bal Yera'eh is the source of the
obligation to get rid of the Chametz once Pesach has arrived, and not
Tashbisu. (This may be Rashi's source for asserting that Bal Yera'eh applies
immediately after Chatzos, as noted in Insights to Daf 4:1.)
It must be noted that even according to Rashi, this does not mean that one
must burn Chametz before Pesach and he may not sell it, eat it, be Mafkir
it, etc. Rather, it means (as the MINCHAS CHINUCH #9 says) that *if* one
decides to destroy his Chametz rather than taking it out of his possession,
the Torah says that he must burn it in order to destroy it totally.
Secondly, even according to Rebbi Yehudah, the Torah also allows one to be
Mevatel his Chametz instead of destroying it (as Rashi said earlier, 4b, DH
b'Vitul b'Alma). Rebbi Yehudah maintains that Tashbisu has both implications
-- to burn it or to be Mevatel it in one's heart (as pointed out by the RAN
on 2a, DH Ela).
(c) The ROSH, however, has a different understanding of Rashi. He says that
according to Rashi's way of explaining Rebbi Yehudah, Chametz can always be
destroyed in any manner. There is only *one hour* during which there is a
Mitzvah to burn Chametz -- during the sixth hour on Erev Pesach. The KORBAN
NESANEL (#8) explains that according to the Rosh's understanding in Rashi,
Rebbi Yehudah's law is a special Rabbinic enactment which is linked to the
Rabbinic enactment to destroy Chametz one hour before Chatzos, and not to
wait for Chatzos. Even though the Gemara presents a Gezeirah Shaveh and a
Binyan Av as a source for Rebbi Yehudah's opinion, the Korban Nesanel says
that these are only allusions which the Rabanan used as a basis for their
enactment to burn Chametz during the sixth hour.
(Note that from Rashi DH she'Im, it does *not* seem that she'Lo b'She'as
Bi'uro is limited to the sixth hour.)