如何理解Vue的.sync修飾符的使用

<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('my-comp', {
 template: '<p @click="increment">點(diǎn)我+1</p>',
 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>