Board Thread:Questions and Answers/@comment-50.153.6.132-20150327064016/@comment-31952814-20150327065001

When each of your unit attacks, the Overdrive gauge fills up bit by bit. Each unit contributes to the Overdrive gauge - using normal attack, BB and/or SBB. When a unit guards, that unit does not contribute to the gauge. The Overdrive gauge is only present when 1 or more units have UBB.

Overdrive gauge is for 7 star units with Ultimate Brave Burst (UBB). To unlock UBB, simply max out SBB to 10. When Overdrive is ready, swipe to the left to activate Overdrive of that unit you are intending to use UBB. Once done, you would need to fill up the UBB gauge, similar to BB and SBB. You only have two turns to activate UBB, so be fast!