New Post: Handling Focus?

hi

Sorry to bump this old post, but I found this answer which solved my problem... but I don't understand one thing :

<TextBox x:Name="UserName" Style="{StaticResource LoginTextBox}">
  <i:Interaction.Behaviors>
    <localBehaviors:FocusBehavior HasInitialFocus="True" IsFocused="{Binding UserNameIsFocused, Mode=TwoWay}"/>
  </i:Interaction.Behaviors>
</TextBox>

Why the following syntax is working :
IsFocused="{Binding UserNameIsFocused, Mode=TwoWay}"

???

I've tried the following, which is a little more understandable and still work :
IsFocused="{Binding UserName.IsFocused, Mode=TwoWay}"

But the following throw an exception, telling me that IsFocused is a ReadOnly property :
IsFocused="{Binding IsFocused, ElementName=UserName, Mode=TwoWay}"

Thanks to the one who can explain this :)