What does 2 Kings 2:10 mean?

Elijah replied, 'That's a tough thing to ask. But if you actually see me when I'm taken from you, then you'll get what you asked for. If not, then you won't.' 2 Kings 2:10 - Modern Text Bible

(Elijah can't guarantee the request, but he gives Elisha a sign: if Elisha witnesses Elijah's departure, it's proof God will grant his wish.)