What does 1 Samuel 6:7 mean?

'So, build a new cart and get two cows that have just had calves and have never been yoked. Hitch the cows to the cart, but take their calves away and pen them up.' 1 Samuel 6:7 - Modern Text Bible

(They plan a test: if the cows, separated from their calves, pull the cart toward Israel, it will be a sign that God is behind all this.)