Latrobe the term flat turn is not interchangeable with break turn even though you often do both together. A break turn is a max performance turn, out of plane with the bandit. If the bandit is wings level then rolling 90 degrees and putting your lift vector on the horizon would be correct because your plane of maneuver (lift vector) is now perpendicular to the bandit's plane of maneuver. If the bandit is flat turning your break turn would be vertical. It's like guns defense, ideally you want your lift vectors perpendicular but anything other than similar is good.
