So when Nakai offered to revalve a couple of dampers for us, we jumped at the chance. There's no better way to learn than by watching a pro and having them give you step-by-step explanations. The process of revalving is the same as rebuilding.
TEIN, like most companies, only revalves their own products. While there may be minor differences between brands of dampers, the principals are still the same. Even if you don't have a set of revalvable dampers, just seeing the process can give you a much better understanding of how a damper works. It certainly did for us. But first, you've got to know a little about how they work.
The Basics
An automotive damper works by providing resistance to the suspension's up and down movement. Without dampers, the suspension can bottom-out, even over the smallest of bumps in the road. And after the suspension slams off the bumpstops, the compressed springs will release all the energy stored in them from the impact of the road imperfection. As the springs release this energy, the car will bounce up again and the whole process will repeat when the car slams back down, making the car jump down the road like it's on a pogo stick. This is why cars with worn dampers or a miss-matched damper/spring combo bounce the way they do.
Dampers provide resistance in the suspension to prevent excessive movement. Damper size and internal valving determine the amount of resistance to movement. This resistance is made by forcing oil through valves and holes inside the damper. As the piston of the damper moves up and down in its case, the piston displaces oil, which must pass through the piston before the shaft of the shock can move. Making it easier for the oil to pass through these holes by making them larger will decrease the resistance created, allowing the shaft to move easier (less damping). Making the holes smaller makes the shaft harder to move (more damping). Simple, right?
While the concept is simple enough, in execution it's a bit trickier. Because cars operate in different conditions and at different speeds, a damper needs more than just one damping characteristic. At high damper piston speeds, more resistance is needed because the forces exerted on the spring and damper are greater. At low piston speeds, cars generally need less damping because of the relatively low-energy impacts the suspension sees.
This is the reason why dampers have "curves." Just like an engine dyno can give us a graph showing engine performance characteristics, a damper dyno can plot out how much resistance a shock provides at different shaft velocities. These "curves" can be adjusted by changing pistons and/or shims.
Now that you know how dampers provide resistance, follow along with the pics and watch how a revalve is performed.
To help illustrate how twin-tube dampers work, TEIN made this cool clear damper. If you look closely, you can see how the assembled shaft/piston assembly shown in the previous picture fits into the inner tube and how the inner tube fits in the main tube. | 
Here's a close-up picture of a piston and shim set. As you can see, the piston has holes in it. As the shaft and piston move up and down in the damper tube, the oil must pass through the holes in the piston. The shims are placed underneath and above the pistons to fine-tune how easily the oil can pass through the whole piston/shim assembly. The shims limit how much fluid reaches the holes in the piston to further control damping. By adding shims of different sizes and shapes, the damper can be tuned to react differently under different conditions. | 
Here's the damper shaft/piston/shim assembly from a monotube damper. Monotubes do not have an inner tube and as such, they can use larger pistons and can use a greater assortment of shims, which allows a greater degree of fine-tuning. |
Here's an assortment of shims and pistons. The larger pistons are for mono-tube dampers and the smaller ones for twin-tube. As you can see, there are a number of different piston and shim designs, which allows a dizzying number of different configurations when you consider that various shims can be stacked and used together. | 
Once the piston and shims have been installed and torqued down, reassembly can begin. Here, the inner tube is inserted into the main tube of the twin-tube damper we disassembled earlier. After a pre-measured amount if oil is poured into the damper, the shaft assembly can be inserted. | 
Once the dampers all back together, all that's left to do is pressurize the damper with nitrogen. The high pressure in the damper helps prevent the oil from cavitating, creating air bubbles in the oil, which in turn upsets the damper tuning when theses air pockets pass through the piston/shim assembly. Here, Nakai charges a monotube damper with high-pressure nitrogen. |