Undertow can be used as a constant slow and a very annoying poke. Many people complain about how Olaf doesn't have any gap close, well by maxing out his Q you're reducing the cool down, increasing the damage, and increasing the slow on this ax. When you pick up the ax the cool down nearly resets itself, allowing you to continue to slow the enemy and hit them like a freight train. I use his Undertow to maintain control over the enemy top laner. And if you ever find yourself in a situation where you have to chase, Undertow allows you to make sure you get that booty. Working similar to a gap close.