How does a component wrapped in @observer know which properties to watch?

Question

JavaScript

Understanding how mobx works internally.
Interested in how the component learns to monitor the property store.testand reacts to changes?

import React, { Component } from 'react';
import { observer } from 'mobx-react';
import store from 'store/index';

export default @observer class extends Component {
  render() {
    return (
      <div>
        {store.test}
      </div>
    );
  }
};