Actually, this is easy to understand. Generally, people with crowded dentition will have such problems when they are unwilling to pull out their teeth.

In fact, you can feel this two millimeters in advance. The easiest way is to chew a piece of gum and put it between your teeth and lips, and squeeze them hard.

That's what it feels like to protrude two millimeters. If you still don't understand, you can correct it by seeing the correction results in advance.