It's controlled by a pendulum.  Any acceleration/ deceleration/ hard leans
left or right will also lock up the reel.
This is how all seatbelt mechanisms have worked since they first installed
inertia reels in production vehicles (since around 1972 or so).

Mike B.

-----Original Message-----
From: Bret Berger
Sent: Thursday, December 30, 2010 2:06 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Pro Tip - Vanagon Seat Belt Inertia Reel

For some future helpless sap to find via Google.

I just spent an hour trying to unwind a the seat belt off an inertia
reel on my 1982 diesel Westfalia which I had removed to lubricate and
wash the seat belt.  The reel will NOT unwind unless it is oriented the
same way it is mounted in the van!  That makes sense... keeps the
(belted) occupant strapped in while the Vanagon is upside down.

BTW, after washing the seat belts in my (front load) washing machine and
giving the inertia reel mechanisms a very light squirt of (non WD-40)
lubricant they work like a charm.

Bret in St. George, Utah