First of all we need to start from a correct SAG: what is the SAG? Read this article now. After you find that the sag is correct, you can work on adjusting the shock.


WHAT IS COMPRESSION?
The compression is a hydraulic braking which has the task of reducing the sinking speed of the suspension which otherwise would constantly crash at the bottom of its stroke. This adjustment is very important because an insufficiently sustained compression leads us to continuous changes in set-up/geometries (and to hit, indirectly, at the end of the stroke) while a fork that is too sustained will not allow us to cushion the bumps well, transmitting every single disconnection to us.
It must be said that, before acting on the adjustments in the event of bottoming out, it is important to act on correct air pressure (or spring).
NOTE: the brands already make a "pre-calibration" in order to allow the user to play with the adjustment but not to find an unusable shock absorber if used at full scale.
By change in set-up and geometry we mean the fact that the suspension, when it sinks, changes the height of the movement from the ground and, indirectly, also changes the steering angle.
DIFFERENT TYPES OF COMPRESSION
Standard: braking occurs constantly regardless of the sinking speed. Unlike forks, in shock absorbers we find lockouts, i.e. a register that leads the suspension to be open or closed (or with an intermediate position). Why this? Because the shock absorber doesn't need the finesse that a fork requires when adjusting: the average user wouldn't have the sensitivity to understand any differences.
High/Low Speed: instead here the sinking speed is taken into consideration because the valve is able to identify how the shock absorber sinks and to intervene in a different way. There are 2 speeds: low and high or commonly called low speed and high speed. We find it in downhill and high-end products.


While in the standard type the adjustment is very simple (with a clockwise click it hardens while clockwise it softens), in the case of high and low speeds (present in many high-end suspensions) the situation becomes more complicated.
WHAT ARE HIGH AND LOW SPEEDS?
Let's forget the term speed in the true sense of the word: low speed does not mean 'when we are going slowly' (how many times is this phrase said in this particular explanation) but when the stresses are light/mild, i.e. the initial part of the sinking in a few words. This adjustment therefore acts on slight compressions, bends in support, braking and oscillations induced by pedaling. By high speeds, we mean the harder hits that cause the shock to move rapidly toward the latter part of the travel. This can happen when landing on a jump, on stony ground or large obstacles in general.


Quizz: if we take a fairly high pavement at 10 km/h do the high speeds or low speeds come into operation? Response: …the high speeds come into action, as mentioned the speed itself has nothing to do with it but the impact felt by the shock absorber counts.
SHOCK ABSORBER COMPRESSION SETTING ADVICE
Try to balance the bike well: if you don't have a real idea of what you're doing, don't set a completely different compression from the fork to the shock.
If you don't know where to start, it's better to open the low and high speeds (towards the -): in this way you will have suspensions that work and from there you will evaluate whether it is appropriate to close them for a moment by acting towards the +.
THE RETURN
The rebound is perhaps the most important adjustment in a suspension because it controls the speed with which the fork/shock absorber extends after the compression phase: if we close the adjuster too much (towards the +) we will find a sluggish suspension that doesn't have time to absorb various close impacts, while if we open it too much (towards the -) we will find an unstable rear that loses precision by extending too quickly (and risks kicking us on bumps or jumps).
Rebound is a hydraulic brake that reduces the rebound rate of the shock. To adjust it, we find a "knob" usually red (conventional) and has several clicks based on the model/brand. The goal is for the suspension to be able to extend between obstacles but without bouncing (too fast rebound) or being compressed (too slow rebound).
The right adjustment lies in the middle and there is no ideal setting for all routes: every self-respecting biker must estimate the rebound variation based on the route.

A clear example to understand the importance of the rebound is to take a beautiful stairway as an advert (naturally downhill): if we face such an obstacle with a too slow rebound we will find ourselves on the third step already flat because the suspension failed to extend completely in the space of time that elapsed between one step and another. In the opposite case, however, an excessively fast return will cause such abrupt re-extensions that the driving sensation in the end will not differ much from that of the previous situation, with a strong instability of the vehicle and "a lot of metal noise (wheels) on ladder” 🙂
To adjust the rebound you need to have a minimum of sensitivity and experience also plays its part: it is in the field that you have to do the definitive test.

SOME ADVICE FOR BEGINNERS
The secret to understanding how the various registers work is to make tests on the same descent, noting the various adjustments and impressions on a sheet (to then keep it as a trophy in your room). To find an optimal value of the return, I suggest at the beginning to make sudden changes on the 'clicks' in order to clearly hear the differences. We start the session at the right half scale (the fateful 0 value) and then we will move to +5 and then -5.
We will immediately understand if we are more inclined for a fast return or for a slower return. At that point we will start from this base to slowly look for the right setup, remembering again that there is no one rebound value that fits everything: for example a faster rebound can go well on technical trails with a lot of rough terrain, while on terrain from a bike park, a slower return is better to have less instability and more safety on bumps or jumps… but then, to conclude, every rider has his preferences!

NOTE: the use of SPD or Flat also changes the suspension setting: in the case of Flat you need to use settings with less "bouncy" (slower return) so as not to lose your feet at the first bump, while with the SPDs you can play on a faster return, but always being careful not to overdo it.
FURTHER LINKS
I recommend watching the following video: The 10 mistakes not to be made in MTB. Here you can find other articles dedicated to maintenance: Heading 365myarrangement
