פירוש על ערלה 2:11

אחר אחרון אני בא – if the leaven of Terumah/heave offering fell last, the entire started dough becomes something that secular and holy things are mixed together, and if something unconsecrated/secular fell in last, everything is permitted, for the last thing is what makes the leavening, for Rabbi Eliezer that permits when that of unconsecrated/secular fell in last, that is specifically when it came first and removed that which is prohibited. And even though that it הועיל a pit that it assists the last that is permitted to complete the leavening. Nevertheless, the taste is made null and it doesn’t come back and get stirred up. But if he didn’t come and remove the prohibited substance and it becomes leavened through both, it is prohibited, for Rabbi Eliezer holds that both this and that causes the prohibition.

Introduction Today’s mishnah discusses leaven of hullin and terumah that fall into dough and neither is sufficient to cause fermentation alone, but together they do cause fermentation.

לעולם אינו אוסר עד שיהא בו כדי לחמץ – for each of them causes that which is permitted, and the Halakha is according to the Sages.

Leaven of hullin and of terumah fell into dough, and neither this was sufficient to cause fermentation nor was that sufficient to cause fermentation, but together they caused [the dough] to ferment:
Rabbi Eliezer says: I go after the last. According to Rabbi Eliezer, whatever is the last to fall in is determinative because that is the leaven that causes the dough to ferment. If terumah leaven falls in first, and then hullin leaven, the dough is permitted because the hullin leaven is what caused fermentation. If the terumah leaven fell in last, then the dough is prohibited because it was forbidden leaven that caused fermentation.

But the sages say: whether the prohibited fell in first or last, it never causes the dough to become prohibited unless there is enough to cause fermentation. The sages are more lenient, and rule that the dough is permitted unless the forbidden leaven is sufficient to cause fermentation alone. If fermentation depends on the hullin leaven, then the dough is permitted.