I just guessed
and it was wrong... close but no cigar.
These adjusters affect the fork damping (not dampening, we are not making it wet) throughout the travel and do not have a predetermined zone where they control the damping.
Speed in low speed and high speed have nothing to do with the speed at which the wheel are traveling down the trail but the speed at which the fork is compressed.
A low speed bump example would be you pushing on the pedals up a hill, a long fast turn where a lot of weight is on the fork for a relatively long period of time, or a big roller coaster type g-out. A high speed bump would be a regular bump/root/rock/curb you encounter on the trail, it is more square edged and when you hit it it will make the fork travel very quickly.
The low speed damping will try and keep the fork higher up in the travel during low speed events so that there is fork travel left if you encounter a high speed bump. In motorcycle road racing it is possible to bottom the suspension in a turn because you are turning so hard, if the suspension is bottomed in that turn and you hit a bump you will bounce and probaly crash. It is a similar concept off road.
When a high speed bump is encountered there are small metal disks that will deflect and blow off the low speed compression and then the high speed takes over. High speed is there to prevent (for example) a 4" travel fork from bottoming on a 4" bump. The reason you don't want this to happen is usually there are more bumps behind that and you will need some suspension left over for the next bumps. The high speed compression helps keep some suspension left over for succesive bumps.
Rebound damping pays a huge role in how a fork feels. If your rebound damping is too slow the fork cannot recover to full travel between bumps and it will get packed down into it's travel. You may incorrectly blame your spring as being too soft or your high speed damping not being high enough. I would say it is safe to say that 95% of the forks I work on have their rebound damping set too high. You want to run as little rebound damping as you can. It difficult to set it perfectly by yourself but you can get close. If you have a partner you will want then to assist in compressing the suspension while you are on the bike. Then release the suspension and watch it closely and note what looks like, if it bounces a couple times like a pogo stick, then increase rebound, if it just comes back to the to and stops, decrease until you get a pogo, then increase a click or two.
The very first thing you need to establish is your spring rate. Most suspension works best with a 20-30% sag. Setting sag can be nearly impossible without a partner. First: Think about what riding position you want your suspension to work correctly. For me: I descend standing and slightly over the front of the bike, I climb standing, slightly over the front of the bike. The only time I am sitting is when the terrain is smooth and rolling or I am getting tired. So I set my suspension up while I am standing in a aggressive position, the result of this is a much higher spring rate in the front and a softer spring rate in the rear. Be sure to include a camel back if you wear one. I see most people setup suspension seated, this results in a overly stiff rear suspension and a soft front.
I went through this description backward. Like Intense1 said above, setting the proper spring rate is the first thing to do, without that you might as well throw everything out the window. Next work on the rebound and get that set. Lastly set the compression adjusters in the middle and go ride. Ideally you will have a shot loop with varying terrain and keep doing laps. Only change one adjuster at a time and maybe only a couple clicks at a time. You want to adjust one direction until the suspension starts to feel bad then go the other way until the suspension feels bad. Then you know your ideal setting is in the middle. Take notes the whole time and so that six months from now you will remember what worked and what did not.
With compression damping manufacturers do not give us much rope to hang ourselves on. You may not notice much difference from full open to full closed. Especially rear shocks, if you look at a rear Fox shock you will see "velocity tune" on the shock next to a bar graph. That is the programed compression damping. There is nothing you can do about that short of sending it back to Fox or to Push. The smaller the bar graph, the lighter the damping.
That is all I have for now, I am sure I will think of more later.