Genèse 38:29

And after the child had withdrawn his hand, behold, his brother came forth, and she said, With what great power hast thou prevailed, and for thee will it be to prevail; for thou wilt possess the kingdom. And she called his name Pharets.

HOW HAST THOU BURST FORTH? THIS BURSTING UPON THYSELF (‘PARATZTA ALECHA’). “What a strong effort you have made!” thus the language of Rashi.
But the word paretz, wherever used, signifies the breaching of a fence and passing through, just as: I will break down (‘p’rotz’) the fence thereof; Why hast Thou broken down (‘paratzta’) her fences? And in the language of the Rabbis: “Pirtzah (a breach in a wall) calleth forth to the thief.” Indeed, the Sacred Language uses the term p’rotz when referring to anything that oversteps its boundary: And thou shalt break forth (‘upharatzta’) to the west, and to the east; And the man broke forth (‘vayiphrotz’) exceedingly., 30:43. Here referring to an unusual increase in wealth. It is thus clear that the word p’rotz is used to refer to anything which breaks forth from its normal boundary. It is for this reason that the verse here is saying, at the time that the first child drew back his hand, and this one hurriedly came out, “What great breach hast thou made in the fence in order to hurry out before him?” The verse says, alecha (upon thee), to indicate that ‘the fence’ was upon him, and he was imprisoned in it. The sense of the verse is thus: “What great breach did you take upon yourself to make in the fence, with the result that you came out of it?”
Rabbi Abraham ibn Ezra said in explanation of the verse: “Mah paratzta? What have you broken, in the manner of a man who makes a breach in a fence and exits through it, and now the responsibility for this breach is upon you.”
There is no point to this interpretation. In the Midrash of Rabbi Nechunya ben Hakaneh. there is mentioned a mystic principle in connection with the name of these children, Peretz and Zerach. Thus they said: “He was called Zerach (shining) on account of the sun which always shines, and Peretz (breaking) on account of the moon which is sometimes dismantled and sometimes whole. Now was not Peretz the firstborn, and yet the sun is greater than the moon? This presents no difficulty, for it does indeed say, And he [Zerach] put out his hand, and it is further written, And afterwards came out his brother.”, thus indicating the importance of his having put out his hand first. Now according to their opinion, the moon is associated with the name Peretz on account of the kingdom of the House of David. Peretz and Zerach were born twins since the moon functions by means of the sun. Thus Peretz is the twin of Zerach who gives forth the hand, while he is the firstborn by virtue of the power of the Supreme One, as is said, I also appoint him first-born. This is the purport of the saying of the Sages with respect to the Sanctification of the Moon: “David King of Israel lives and exists.” The man learned [in the mystic teachings of the Cabala] will understand.

ויהי כמשיב ידו, the prefix כ meaning “as if,” describes that he did not really retract his hand. He was forced to, being pushed by his brother.

ויהי כמשיב ידו, at the time when he withdrew his hand his brother pushed ahead and emerged first. We have a similar construction to this in 40,10 והיא כפורחת, which also describes a process which was deceptive. [that which blossoms first does not always ripen first. Ed.]

ותאמר, these words were addressed by the midwife to the newly born infant who had pushed to displace his brother.
פרצת, why have you burst forth from the confines of the womb displacing your brother who had been in the process of emerging?

עליך פרץ, you have acted precipitously, inappropriately, in claiming something which was not meant to be yours, the birthright.

ויקרא, Yehudah called his name Peretz in agreement with the comments of the midwife.