The Bigger the Tree, the Harder it Falls.
An upset occurs when a player/team of higher caliber loses to the less skilled player/team, therefore upsetting anyone who expected the win to go by the percentages. Sometimes, upsets get so ugly that fans, particularly of the home team, will riot after a horrible loss. Other times, the mass majority of onlookers and spectators are anything but upset and will actually celebrate the underdog who miraculously pulled a victory from the jaws of expected defeat. Everyone loves a David and Goliath story. Well, except for Goliath and his army.
Valuable lessons can be gathered from studying upsets. First of all, if you are the higher ranked player who is expected to win, be extremely vigilant. Your opponent may come in with the feeling that he/she has nothing to lose and play with reckless abandon. Before you know it, you are getting run over and can't stop the momentum of a carefree spirit who you unwittingly gave all the momentum. While you are expected to win, try to treat the opposition with the same respect you would against a higher ranked opponent. This is easier said than done because the same level of fear is not present. One way to generate the level of motivational fear you need is to imagine what would happen if you lost the game. Use that feeling of anger and despair to spur yourself on to trying even harder, despite (and, essentially, because of) your lower level of opposition.
If you are the lower ranked player, recognize that you have the advantage of surprise on your side. Your opponent may likely have two feelings which you can capitalize on: overconfidence or fear of losing. In either case, you must be extremely watchful for the first opportunity to pounce on a mistake and steal the momentum. Once that happens, do not get excited. Act like you've been there before. Most underdogs waste emotional intensity when they get the advantage because they themselves did not expect to win and are now feeling excited about the prospect of victory. Do not forget that your opponent is now a cornered animal who will do anything possible to rip the game out of your hands. Keep your energies focused solely on the next move, not the potential result, which will only come if you do all the right things step by step anyway. And even if things turn against you, don't ever give up. Upsets are usually rooted in emotional mistakes by the better player or team, and that can creep back into a game at any time. Just remember: it's not over till the fat lady sings.
Below is a game where Viswanathan Anand lost to Olivier Touzane, a player 402 points below him.