如何理解Vue的.sync修饰符的使用

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