Or HaChaim on Genesis 38:28/אור החיים על בראשית ל״ח:כ״ח

1 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.