"Most martial artists get illusions in training,” he said. For example, if you are trained as a boxer, you come to trust that an opponent won’t try to tackle you to the ground—because that’s not what people do in boxing rings. Instead they hit each other, with gloves on (another condition that is not very realistic).
BJJ also assumes a set of rules, not unlike boxing, wrestling or karate. You're not allowed to take a bite out of your opponents arm while they are choking you, or pull their ear off, or stick your thumb in their eye.
BJJ is also often practiced wearing a gi which allows for different techniques than if your opponent were wearing say, a t-shirt (or no shirt at all).