וַיְהִ֥י בְלִדְתָּ֖הּ וַיִּתֶּן־יָ֑ד וַתִּקַּ֣ח הַמְיַלֶּ֗דֶת וַתִּקְשֹׁ֨ר עַל־יָד֤וֹ שָׁנִי֙ לֵאמֹ֔ר זֶ֖ה יָצָ֥א רִאשֹׁנָֽה׃
她生產的時候，一個孩子伸出一隻手來，接生婦就拿一條朱紅線綁在他手上，說：「這是先出來的。」
Rashi
ויתן יד THE ONE PUT OUT HIS HAND — one stretched forth his hand outside, and after she had bound the scarlet thread upon his hand he drew it back.
Radak
ויהי בלדתה ויתן יד, one of them stuck his hand out of his mother’s womb, signaling that he wanted out. Seeing that he did not succeed, he withdrew his hand.
Or HaChaim
על ידו שני לאמור, a scarlet thread on his hand, as though to say, etc. The midwife had a sudden flash of Holy Spirit and did something without being aware of its true significance. By tying Shani scarlet thread around his hand she indicated that the baby was Sheyni the one born second, though it had left the womb first. The Torah adds the word לאמור, "meaning to say" that this one emerged first. Although the midwife said so the truth was the reverse. The red thread was to prove יש אם למסורת, that there is a distinct significance to the spelling of a text as well as to the accepted way of reading a text.