I believe I.S. barrier is activated manually by the pilot, I can be wrong though. (Absolute Defense is activated automatically when Barrier can't do jack)
Maybe there's multiple type of barrier to keep I.S. more aerodynamic than they appear, though not perfect either. It was also implied that it was designed for space operation, despite the lack of helmet and torso armor and exposed arms and legs, suggesting that I.S. using some sort of barrier to keep a normal atmosphere inside. (never mind that in space combat, that's equal suicide anyway).
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