Shemos 18:9

ַיִּ֣חַדְּ יִתְר֔וֹ עַ֚ל כׇּל־הַטּוֹבָ֔ה אֲשֶׁר־עָשָׂ֥ה יְהֹוָ֖ה לְיִשְׂרָאֵ֑ל אֲשֶׁ֥ר הִצִּיל֖וֹ מִיַּ֥ד מִצְרָֽיִם

“…who saved them from the hand of Egypt.”

Rashi explained, citing Mechilta, until now no slave was able to flee from Egypt. A learned man asked me from that which our Rabbis of blessed memory said (Shemos Rabbah 20, 11) that the children of Ephraim went out before the time and were killed by Philistines.

After a long time, I found this question raised in Yalkut David (Parshas Bo, on the verse And the children of Israel went up armed) and he answered that for the honor of Joseph, they did not enslave his children, and therefore they did not prevent them also from leaving.

And this is a new thing, for our Rabbis of blessed memory said (Shemos Rabbah 5, 15) only the tribe of Levi was not enslaved.

And in my humble opinion, it appears according to what is known in the holy books that the entire grip of Egypt on Israel was by the power of the impurity of adultery that ruled in their land (Vayikra Rabbah 23, 7) which for this they were called (Bereishis Rabbah 42,9) “nakedness of the land.”

And according to this, Ephraim is the opposite of forbidden sexual relations, just as the Gaon, the Holy Rabbi Tzadok of Lublin, wrote (Likkutei Maamarim page 69 and Tzidkas HaTzadik 159) that every place mentioned in Tanakh regarding any man, the title “Ephrati,” the intent is that he never issued a keri emission in his life.

And therefore the children of Israel are called by the name Ephraim (Yermiyahu 31:20) “Is Ephraim my dear son,” for they flip the 6ycombination Kuf-Reish-Yud (Keri/Emission) to the combination Yud-Kuf-Reish (Yakar/Dear), therefore the Egyptians had dominion over the children of Ephraim.