How does __proto__ work when object is created with Object.create(null)

Consider the following javascript code

var a = Object.create(null);
a.foo = 1;
var b = Object.create(a);
console.log(b.foo);  //prints 1
console.log(b.__proto__);  //prints undefined
b.__proto__ = null;
console.log(b.__proto__);  //prints null
console.log(b.foo);  //prints 1

Can anyone explain how object b is accessing the "foo" property of a even after setting b.__proto__ to null? What is the internal link which is used to access the property of a?

I tried searching through SO for possible explanations but couldn't find any explanation for this particular behaviour of Javascript.