Here's a picture of the internals of a seatbelt that kept jamming, in the middle rear seat of my 2000 Volkswagen GTI:

In an accident, the little ball is supposed to throw the black catch against the red cog, to stop the belt from rolling out. If this mechanism gets stuck, no amount of force and swearing is a match for the force of an accident, which the mechanism is designed to endure. So one's only hope is to coax it loose. To do so, one needs an idea how it works.

Potamkin Volkswagen in NYC (otherwise excellent service department) made two attempts to fix my jamming problem, the first time selling me a new seat belt mechanism as a matter of course. The problem immediately reappeared with the new seatbelt mechanism. I have since moved the car to CA, and ended up taking apart the seatbelt to see what is going on. In hindsight it wasn't the ripoff it might seem for them to sell me a new mechanism, as the mechanism costs far less than labor.

As is so often the case in $20,000 and up cars, one can be tormented by pieces of plastic worth pennies, but which require hundreds of dollars of labor to access. Here, after seeing what is going on, I can no longer blame Potamkin, they didn't design the damn thing. Each component of the rear seat system may be well-engineered, but as a system it is poorly engineered.

Once jammed, this mechanism can only be coaxed loose when the seat is in the upright position, and after the belt is let in half a tick of the red cog's teeth. Then gently pummel the seat to coaxe the black catch into falling. It may be necessary to remove the plastic seat belt guide to generate half a tick of slack, e.g. if the seat belt jams while the seat has been folded down for cargo. This is the design flaw in the system.

In the worst case one needs to disassemble the seat. Even then, it is important to understand the role of gravity in unsticking the mechanism. Don't expect any help whatsoever from Volkswagen's $100 service manual on disassembling the seat; you might as well just wing it.

One could generalize from the lessons of this example:

1. Never force any uncooperative part; this will generally just damage it. Instead figure out how to tease it into cooperation.
2. Before taking anything apart, order a replacement part, and destroy it to figure out what's up.

For example, the rear headrest guides also self-destruct quite easily if forced. One should buy a spare set at the time of purchase of a new car, and study and play with them to understand what to do and what not to do when inserting and removing headrests.