As he drew back his hand: that is, “As he pulled his hand back inside the birth canal.”

Behold introduces a surprise event, which is: his brother came out. See Good News Translation.

What a breach you have made for yourself is literally “How you have torn a break [a place broken open] for yourself.” Breach in Hebrew is parets, spelled Perez in English. See Revised Standard Version and Good News Translation footnotes.

Therefore his name was called Perez: or “Therefore they called him Perez.”

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .