silvia test pipe in video... ?

http://www.allfours.org.au/videos/Drift%20Club%202000.mpg

in this video, which is 599mb long (awesome quality, big video, 59 minutes long), there is a section at the end with some preferred parts for drifters...
there's a test pipe that they show that's pretty interesting.
it's going on an s15 silvia, and it's 3" diam.

there's an "insert" of sorts that goes into the front of it, and it's about 1/2 the length of the test pipe. it steps down to about 2 1/2" diameter, but just ends.

any ideas as to why you'd want something like that? specifically on a drift car?

i'd post this in the 240 or silvia forum, but i'm sure the same guys hit both places...

i'm interested in that, i've never seen anything like it...

i havent even dl'd the complete vid yet. but thanks
:D

it looks like a restrictor, or it could be a silencer