Ewert explained this in field lingo at the 1997 Wilderness Risk Managers' Conference. As the water tapered into Class 2 conditions, they narrow their awareness to what's immediately in front of them. They notice the waves they may have to negotiate, and they are talking to their partner, but they aren't seeing the grizzly bear on the bank or the developing storm above them. As the water turns to Class 3 conditions, they can barely hear what their partner is saying, and don't notice that it's raining until the thunder gets loud. This canoeist who is over his head in whitewater, can't even perceive everything that is going on, much less integrate it into an informed decision.
Stress has an optimum level for learning anything.
Too little and we aren't motivated. The student learns to live with the natural elements rather than against them. One must learn to master the things he is capable of controlling and to bend with the elements he cannot control. A rash decision or simple failure of equipment is capable of precipitating disaster if one does not have the ability and the judgment to accept his limitations. The best way to learn judgment is the best way to learn anything.