It appears to be a valve that vents intake to the atmosphere.
A dump vent. The mpg movie at the site made it easier to see
this (http://www.spacers.nu/ down at the bottom). You can see
the valve moving and venting air as the car shifts gears.

Also, this strikes me as a bad idea since the intake has been metered
by the fuel system and the turbo has a built-in bypass
valve that serves the same purpose (although these add in valves
might make that "cool" swish sound).

The idea here is that when you shift, or back off the throttle
for a turn, the throttle plate closes. This slams the door on the
turbo and can actually send a shock wave back to the turbo (in addition
to slowing it down). Bypass valves are used to vent to the
atmosphere when the throttle closes. This keeps the turbo
spinning and you get less turbo lag when you get back on the
gas. As mentioned, the mitsubishi turbos used on 850/70s have
a bypass valve built in the opens and connects the compresspr side
back to the intake (keeping the turbo spinning). Im not sure
if the LPT engines have this; but the HPT engines do.