Motorcycle suspension has two important parts: springs, which absorb energy from bumps, and damping circuits that convert that energy to heat instead of to uncontrolled rocking.
Your damping circuits have two separate parts, a compression damping circuit and a rebound damping circuit. Generally, these systems work by having a piston move through oil and a small hole for the oil to pass through. The oil passing through the hole makes the resistance that absorbs and dissipates the energy from the spring.
Oil passing through a hole has resistance that is proportional to velocity squared. You know another system that has resistance proportional to velocity squared: a parachute. More or less, no matter how much you weigh, you're going to fall with a parachute at about 10-12 mph. Square law devices act as speed limiters.
There are a lot of solutions to this problem these days, but they all more or less boil down to one thing: use a spring to make the hole larger or smaller depending on oil pressure, thereby making a system where the resistance is proportional to velocity instead of velocity squared. Cartridge forks do this with thin washers that deflect to open up the oil hole; Works Performance shocks have ball bearings that are spring loaded to open up oil passages; RaceTech Gold Valves have spring loaded pistons that open up oil passages.
Some years ago it was found that in Supercross (motocross with ginormous jumps) that springs that were strong enough to handle the jumps were so strong that on normal bumps, the suspension basically didn't move. Springs that worked well on the normal stuff bottomed out immediately after a big jump and didn't absorb enough energy. So the racing guys came up with the idea of using the square law to limit suspension speed. Supercross suspension uses springs for the small stuff, and square law damping for the really big stuff. However, if you never get more than six feet in the air, you have no need for this stuff. The same square law damping that makes Supercross bikes work is the stuff that makes normal suspension harsh over square edged bumps. For reasons which I find incomprehensible, nearly all suspension made in Japan now has this square law damping built in to do speed limiting, even on bikes like large Harleys that will never get two inches into the air, much less twenty feet.
This is the procedure to get the square law damping under control in most conventional forks, and thereby allow you to set the rebound damping for better performance without having to put up with a harsh ride. We'll illustrate this procedure on a set of Suzuki VStrom DL650 forks. Note: Cartridge forks are enormously more complicated inside than conventional forks, and should not be taken apart unless you have been trained. There's lots of little tiny pieces, and special tools are required. Most all upside-down forks are cartridge forks. If you have these and have a problem, I recommend you contact the guys at RaceTech.com.
Your damping is set by three things: the oil viscosity in the forks, the diameter of the compression damping holes, and the diameter of the rebound damping holes. We would like to increase the rebound damping, which we can do simply by increasing the oil viscosity. Most forks are shipped from the factory with incredibly cheap 5w oil. However, if we simply increase the oil viscosity, we'll also increase the compression damping and the forks will become more harsh. So we'll first increase the diameter of the compression damping holes. In the damper rod shown below, the compression damping hole is about .2" in diameter and is near the right hand end. The rebound damping hole is about 1/16" diameter and is close to the piston on the left.
We clamp the damper rod in a vise and drill out the compression holes. There are two holes on this rod, one on each side. Sometimes there are four holes. Here the top rod has been drilled out to 3/8" diameter.