An en fine guy would know, but it was my understanding that the air was pretty much just bypassed around all the usual turbine stuff and rammed into the combustion chamber.
Almost like you could take away all the turbine bits in the high speed mode and it would happily chug along.
Then again, I could well have it all wrong about the bypass plumbing. Have to admit on seeing the real plumbing for the BP lines, they did not look streamlined enough for the airglow volume.