AngularJS: update the model in parent scope from inside the

Question

i am new in angular and learning. when i see the code from this url http://stackoverflow.com/a/19177773/6188148 then things is not clear.

see the code and tell me why model data can not be updated from child scope ?

<input type="text" ng-model="data">
<div ng-if="true">
<input type="text" ng-model="data">
</div>

see the below code again and tell me what special things is there for which model in parent scope is updating from inside the child scope ?

<input type="text" ng-model="data.input">
<div ng-if="true">
<input type="text" ng-model="data.input">
</div>

they use just data.input and parent data is updated from child scope....this is not clear. what is so special in .input keyword ?

please help me to understand how second example can update parent data just by .input keyword ?

also tell me when child scope gets created ? when we use ng-if or ng-show then ?

thanks