Fog: decisions from the radar and the ear
You cannot see the other ship: you have an echo on the radar or a fog signal in your ears. While the vessels are not in sight of one another, nobody gives way and nobody stands on: both owe the early, bold manoeuvre of Rule 19.
What you practise here
The manoeuvres of Rule 19: which way to ease away from an echo, when to slow to bare steerageway, what a fog signal is telling you. The radar shows true vectors and trails, and the signals play like a real whistle (the full signals list). Good visibility is trained in the "give way or stand on" situations.