Es6 Generators are little confusing me.How its working here?

I knew, how i got { value:6, done:false } at first time.But,next values how can it be { value:8, done:false } and { value:42, done:true }

//-----------code-----------//

function* foo(x) {
    var y = 2 * (yield (x + 1));
    var z = yield (y / 3);
    return ('OP'+x + y + z);
}

var it = foo( 5 );

// note: not sending anything into `next()` here
console.log( it.next() );       // { value:6, done:false }
console.log( it.next( 12 ) );   // { value:8, done:false }
console.log( it.next( 13 ) );   // { value:42, done:true }