ונשל הברזל, the actual axe is called here ברזל, iron, by the Torah, as distinct from its wooden handle. We find the word ברזל used in the same sense in Kohelet 10,10 ואם קהה הברזל, “if the axe is blunt.” The scenario described by our verse is that the axe while hitting the wood it splits sends a piece of that wood sailing through the air, with fatal consequences for the victim it hits. This is clearly something that was entirely unplanned by the person swinging the axe.

פן ירדוף, these words refer back to the need to establish three such cities as outlined in verse 2. If there were to be only one such city of refuge, the avenger in his pursuit would be more likely to succeed.

ואם ירחיב ה' אלוקיך את גבולך, this paragraph speaks of future events, a time when G’d would give the Israelites also the territory of the remaining three Canaanite tribes mentioned in the covenant at the pieces in Genesis, 15, the first covenant made between G’d and Avraham.

לא תסיג, the word is similar in meaning to Isaiah 42,17 נסוגו אחור, “they retreat backwards.” Our verse describes someone who adjusts his neighbour’s boundary backwards in order to increase the size of his own land.