It says in the Gemorah in Shabbos 141a that one may crush Palpelei one by one with the handle of a knife.
The reason this is muter is because it's being done with a shinui. However normally we find that when a malacha is done with a shinui it's still ossur however perhaps only m'drabbanan. However in this case we find that it's muter completely?
Firt of all, what's p'shat in this? And second if we find this anywhere else that al yadei a shinui we can be matir a malacha to do seemingly even lichatchila.
Yehoshua, Yerushalayim, Eretz Yisrael
1) The Beis Yosef (Tur, Orach Chayim 321, DH Kasav ha'Agur) asks this question.
He answers in the name of Rabeinu Yeshayah that crushing with the handle of the knife is considered a "Shinui Gamur"; a totally different way of doing it.
Rabeinu Yeshayah cites the Mishnah in Beitzah 14a, where Beis Hillel states that on Yom Tov one may crush spices in the normal way with a stone pestle, while salt may be crushed only with a wooden pestle. Rashi writes that the salt must be crushed in an unusual way, but a wooden pestle is sufficient, even though this is only a minor Shinuy.
We learn from the above Mishnah that a wooden instrument is considered a Shinuy. Rabeinu Yeshayah adds that if one does it with the handle of the knife, this transforms it into a Shinuy Gamur and is therefore permitted l'Chatchilah.
2) Rabeinu Yeshayah cites another example of this idea from Shabbos 74a. Doing "Borer" with a "Napah" or "Kevarah" is forbidden mid'Oraisa because the standard way of selecting is with these utensils. Doing "Borer" with a "Kenon" or "Tamchuy" is forbidden mid'Rabanan because doing Borer with these utensils is a Shinuy, not the standard way. If one does Borer with one's hands, this is permitted l'Chatchilah, because it is a Shinuy Gamur.
3) I heard of an example of Shinuy Gamur -- in a different area of Halachah -- from Rav Levi Yitzchak Halperin shlit'a, the Director of the Institute for Science and Halachah. The Shulchan Aruch, Orach Chayim 32:5, writes that the Parshiyos of Tefilin must be written with one's right hand. If one wrote them with the left hand they are Pasul, but if it is impossible to find any other Parshiyos, these may be used but a Berachah should not be said on the Tefilin. The Mishnah Berurah (32:19) cites the Magen Avraham who says that if one held the quill in one's mouth and wrote the Parshiyos, they are Pasul even if no other Parshiyos are available, because it is a totally different way of writing.
Kol Tuv,
Dovid Bloom