Sigma electron movement in resonance forms

I have come across an example in my textbook, which shows resonance structures for the bromonium ion. However, I am confused as to why the sigma bond electrons "move" to get the tertiary carbocation and the secondary carbocation. Are sigma bond electrons not strictly localized and pi electrons delocalized?


[![enter image description here](https://i.stack.imgur.com/WTHQS.png)](https://i.stack.imgur.com/WTHQS.png)
My question is: **When are we allowed to move the sigma electrons when writing resonance forms?** 


