I'm confused by the last statement. As holding :f while backturned in 3.1 took longer than 5 frames to turn around. This is easily proven by Bass' back-turned pickup granting him +11 and his
3
(17th frame is the hit frame) being guaranteed to hit if you just hold :f as the defender.
In DOA4 it turns around in 2 frames, but it did give an animation when you SE while back turned, which was new.