That's another aspect that I sort of love about the scoring system. Besides the addition or subtraction of points for each movement, depending on its quality (great/correct/not-so-great), there are also point deductions for deviations within the pattern.
If it's a minor deviation -- like over or under spinning by one-eighth (up to one-quarter) of a turn, a delayed change of lead, break of gait, failure to pass the markers denoting where a stop is supposed to occur, etc.... -- these can result in deductions of anywhere from a half-point to 2 points. These are minor penalties, but they can add up pretty quickly. Accuracy is the name of the game in reining.
A major deviation is something along the lines of the horse bucking or rearing during the pattern, the rider grabbing hold of the saddle or touching the horse with his/her free hand... This will result in a 5 point deduction. You really want to avoid this type of penalty as a rider. And you can get multiple penalties like this. Say I grab hold of the saddle for a brief instant and let go; I get an automatic deduction. But if I were to grab hold again, it's a whole new deduction added on to that. In some ways, I might as well keep holding on once I've done so -- it'll only result in the one deduction.
A 0 Score is basically the result of a the most major deviations. This could be:
- the use of two hands: if you're supposed to be going one-handed (there are classes for green horses or green riders where you can actually go two-handed), even simply touching the reins with your other hand is an automatic 0.
- going off-pattern: say you make a mistake and go the wrong direction to start -- you're done. You just went off-pattern and scored a 0. And this is surprisingly easier to do than it sounds, as the patterns are so alike with such minor differences that you can simply start riding the wrong pattern out of habit. Or you might simply do something as silly as executing the rollback the wrong way.
- over-spinning by more than a quarter of a turn: this is one you see ALL THE TIME. Sometimes you forget to count properly, sometimes you are so focused on correcting an issue that you accidentally allow it to happen, and frequently you simply spin so fast that you just make a simple mistake and spin five times instead of four. It happens A LOT.
Frankly, I wouldn't hate seeing this being adopted by the dressage community. They'd never do it, but I like the idea of it. There should be definite instances where the rider/horse combination is clearly off-pattern, for whatever reason, and the chance of getting a 0 score serves to make you that much more honest. Reining clearly defines itself as a sport in which there is no resistance, the horse is willingly guided. If you go off-pattern at all, if you break gait for longer than half a circle, go backwards for more than two strides....these are all signs of resistance and will result in a 0 score, as well it should. Why we wouldn't adopt a similar attitude in dressage is beyond me. Dressage purports to be THE sport in which horse and rider work together in teamwork, without any resistance. Why wouldn't we be just as stringent in our requirements for demonstrating that?