Dan,

That is for a needle valve style boost controller.
The inner fitting gets a pressure signal and the outer fitting gets the same signal so that there is no movement. When the boost is the right level it just closes the needle valve slowly until the rate of air intering the outer fitting is balance with the rate of leakage out the vent corresponding to the desired boost level.

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black