One of the interesting things about fencing is that not only does it combine fine and gross motor skills like many sports, but each is also asked to draw on a huge library of skills in the middle of the bout. To mitigate this, coaches will teach the fencers to focus on 2-3 key actions, actions with over 80% accuracy that will ensure victory... but what if they don't work? If they don't work, the fencer must go back to that library, which is only possible if he has the presence of mind to analyze the situation on the fly, and either replicate a solution from previous experience - or create one.
The problem with this is that there is a person on the other end of the strip with a metal weapon in his hand, and he's coming for you. Only the quarterback in tackle football, and maybe the defensive secondary unit faces a similar dilemma - how to match solutions from a wide body of knowledge to a specific high pressure situation instantaeously. Most folks in such a situation freeze up because they can only see what is directly in front of them - hence "tunnel vision."
Tunnel vision is a phenomenon of the mind and body working in concert against an athlete's, or anyone's welfare. It's overfocusing on the most immediate issue, instead of taking in as much information as possible and making an informed decision. If you are running for your life in an open field - tunnel vision is ok. If you are running for your life in a jungle, the jaguar on the tree above you can get you just as good as the one on your tail. But if your field of vision is clear, and your mind is open - you can avoid them both.
When coaching from the sidelines, especially in fencing, sometimes it's not as important to convince a competent fencer to try a certain action as to get him to see the whole picture. If a 4-8 parry riposte will be more effective than a simple 4 parry-riposte - telling him might help, if he can see when to set it up. But if he has tunnel vision, that may not be possible.
Some of my coaching, like "Walk the dog" is less a suggestion on what to do, but a way of mentally pulling the fencer out of the chase. His body is still on guard with weapon at the ready, his drilling and training have made immediate parries automatic - but instead of being consumed by the madness of the moment, his conscious mind has now regained control and his eyes are able to see the entire field instead of the fencer that has confounded him. Much like the Pro Bowl level quarterbacks able to pick the perfect spot and receiver to pass the ball to as men the size and speed of rocketpowered refridgerators try to smash him - so can a fencer with a completely clear mind execute common and uncommon actions with ease. After all if the fencer has done his job in practice, somewhere in his mind he has the solution to victory - a coach need only help him clear his mind so he can find it on his own.