As a basic outline:
A beginner motocrosser wants the bike to give a lot of good feedback, minimal bad feedback (harshness), swallow bumps at a moderate speed, and most importantly, give the rider confidence with a nice balanced feel. This is achieved by utilizing a somewhat stiffer low speed compression stack, in conjunction with a somewhat softer high speed compression stack, This gives good feedback, and allows the suspension to ride higher in it's stroke, given the motorcycle's average speed. This valving usually uses a high-flow piston. This set-up is somewhat similar to a stiff off-road set-up.
An intermediate motocrosser wants their bike to have more bottoming resistance than a beginner set-up. This rider also wants a lot of good feedback, minimal bad feedback (harshness), yet the rider wants the suspension to feel plush and give he/she good confidence with a nicely balanced feel. This is achieved by utilizing a somewhat softer low speed compression stack, and a somewhat stiffer high speed stack. This gives a plusher feel, more bottoming resistance, given the motorcycle's average speed. This valving may use a high-flow or a low-flow piston, depending on the required bottoming resistance.
A professional motocrosser most importantly wants the bike to have great bottoming resistance at speed. The balance between plushness and bottoming resistance is usually calibrated to the riders preference. A lot of times some "small bump" plushness may be sacraficed. Some pro riders also prefer less low speed compression damping to help certain riding styles and to help "seat bouncing/staying low on jumps" etc. A lot of times the low speed compression valving is utilized to somewhat "soften the blow of harshness"caused by the extreme high speed compression valving, or piston porting. This type of valving is usually only good for its designated rider(s), and is usually quite stiff for the averate rider, especially for off-road. This style of valving usually uses a low-flow piston.
Sandy tracks usually require stiffer high and low speed compression damping/valving and stiffer high and low speed rebound damping/valving, regardless of the skill level of the rider, motocross or off-road. Sand tends to act very "springy". A hard-packed moto set-up is usually too soft for sandy tracks.
This is just an outline, as some riders actually prefer, and/or go faster on suspension calibrated slightly different from the norm. Every rider and track is different. Valving testing is the only way to tell.
High-flow pistons require completely different overall valving than what a Low-flow piston design would. High flow piston=stiffer valving, Low flow piston=softer valving. A Low-flow piston is a "controlled high-speed packing device", and a High-flow piston uses shims for it's high-speed valving.
As a bacic outline, there is basically two different styles of off-road valving set-ups. One is fairly soft, the other is fairly stiff, basically depending on how fast, and/or how rocky or whooped out the conditions are. Both usually use a high-flow piston, to decrease harshness and to be more versatile. Both are valved to the rider's ability.
Enduro and or harescramble riders want a suspension set-up that is plush and doesn't wear out the rider at the end of the race. This valving tends to be on the soft side. The low speed compression is set somewhat soft, as is the high speed compression. The low speed is set stiff as possible, untill the initial plushness is sacraficed, then it is valved one notch softer. At high shaft speed/low shaft travel (ei. small rocks, roots etc.) the low speed compression stack MUST move, or the bike will deflect on slimy, squared edged impacts. This is WAY more important on an enduro/harescramble bike than a motocross bike! The low speed compression is stiff enough to hold up the bike during braking, and keep the suspension up in it's travel. The suspension is soft enough to be initially plush, and allow the rider to use minimal input to transfer weight while retaining proper wheel pressure. The high speed compression valving is set-up fairly soft, but just stiff enough to handle average sized whoops. This is a welcome trade-off, in order to be extremely supple and swallow roots and rocks. This style of valving is extremely effective in long gnarly races where minimal rider input is key.
High speed GNCC and/or desert riders want a set-up that won't get mushy in the sand, but be fairly plush for long races. This style of valving tends to have a bit more low speed compression valving, to give the rider more "feel" and feedback which is required at faster speeds. The high speed compression valving is also stiffer, to deal with bigger impacts at faster speeds. This set-up is somewhat similar to a motocross set-up, but with a bit less high speed compression damping.
These are just generic outlines for valving configurations, so you get the basic idea how it works. Rider skill level, rider weight, rider height, and tracks/conditions vary, as does the valving requirements.
Ride Concepts Inc. uses a data aquasition system called a "Shock Clock". This is an electronic device that mounts to your bike as you ride. It senses the amount of travel being used vs. time. It's memory is downloadable to our computers, to help diagnose/set-up custom valving stacks.