first time u use overload the bar will appear to not have changed, just open skills window and setup what skills u want to use while in overload, when u cancel overload the bar will change to what it was before u started overload, every time u enter overload after the bar will change to what u setup while in overload automatically
hope thats clear

similar to setting up skill bar for werewolf except overload copies your current skills so it dosnt look like the bar changed.

When you cast Overload your bar switches to the third bar. It's a bit subtle.