TB Bekhorot 33b What’s the difference between דָּבָר שֶׁאֵינוֹ מִתְכַּוֵּין and pesik raisha?

I fully didn’t really understand the difference between דָּבָר שֶׁאֵינוֹ מִתְכַּוֵּין and פָסִיק רֵישָא וְלֹא יָמוּת until Art Scroll elucidated them. Remember one may not actively create a blemish on the firstborn animal. The case under discussion is may a person blood let a firstborn animal that was congested with excess blood, even if the animal will die if one does not let the excess blood?

“Rabbi Shimon permits one to blood let the firstborn animal even though it will cause a blemish. “Rabbi Shimon’s ruling is based on his general position thatדָּבָר שֶׁאֵינוֹ מִתְכַּוֵּין, an unintended act is permitted. That is, a person may carry out a permissible act, even if-as an unintended consequence-a second, forbidden act might also be performed. [For example, one who drags the chair across the lawn might make a furrow in the ground (which is forbidden on the Sabbath under the category of plowing). Rabbi Shimon permits dragging the chair, since the person’s intent is to move the furniture, not to make a furrow.] In our case, Rabbi Shimon permits bloodletting although a blemish might result, because it is not the Kohen’s intent to render the animal permitted for his own use by blemishing it; rather; his focus is on saving the animals life. {The Sages and Rabbi Yehuda, who prohibit bloodletting, hold that and “unintended” act is forbidden.]

“[Rabbi Sherman permits an “unintended” act only if the forbidden consequences is a possible result, not an inevitable one (פָסִיק רֵישָא וְלֹא יָמוּת). Thus, Rabbi Shimon’s dispensation in our Mishna is limited to where the bloodletting might-but will not necessarily-inflict a blemish. If it will certainly inflict one, Rabbi Shimon agrees that it is forbidden.” (Art Scroll, TB Bekhorot 33b2, note 13)