Vue의 .sync 수정자 사용을 이해하는 방법

<p id="app">
  <p>{{bar}}</p>
  <my-comp :foo.sync="bar"></my-comp>
  <!-- <my-comp :foo="bar" @update:foo="val => bar = val"></my-comp> -->
</p>
<script>
  Vue.component(&#39;my-comp&#39;, {
    template: &#39;<p @click="increment">点我+1</p>&#39;,
    data: function() {
      return {copyFoo: this.foo}
    },
    props: [&#39;foo&#39;],
    methods: {
      increment: function() {
        this.$emit(&#39;update:foo&#39;, ++this.copyFoo);
      }
    }
  });
  new Vue({
    el: &#39;#app&#39;,
    data: {bar: 0}
  });
</script>