The meaning of guarding the door

Experts look at the doorway;

For example, on the road, you suddenly see that a person's car engine is broken, so you can repair it. When you are repairing the car, many irrelevant people will come to see your jokes. Why is this man so nosy? It's sheer bluff to see if he can repair it.

You are an expert here, and others just watch you busy.

The meaning of watching the doorway;

That is to say:

People who don't understand a problem, look at the surface, look at the general. People who are experts will see the essence of the problem clearly and see where the problem lies.

At the same time, "watching the doorway" means: look at the crux of the problem, that is, the essence, and then prescribe the right medicine.