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


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 fork if used at full scale.
By change in set-up and geometry, we mean the fact that the fork, when it sinks, has a head angle that closes, so we understand how important it is not to have a fork that is continuously "packed". Let's think of a corner in support: if we have a well calibrated fork we will come out fast (with the right technique), if we have a badly calibrated fork we risk closing the front!
DIFFERENT TYPES OF COMPRESSION
Standard: braking occurs constantly regardless of the sinking speed.
High/Low Speed: instead here the sinking speed counts because the valve is able to identify how the fork sinks and to intervene in a different way. There are 2 speeds: low and high or commonly called low speed and high speed.
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.
WHY DON'T I HAVE A COMPRESSION ADJUSTMENT?
Because either you have a basic fork model or you have a model that allows you to use pre-set positions, such as the Fox CTD (with manual lever or remote control). It is nothing more than allowing the user to go and harden the fork with a single click, therefore a facilitated compression adjustment system.
This system is widely used in more pedalable bikes (Cross Country/Trail) because it is preferable to have the possibility of blocking the bike for pedaling rather than having a very precise and fine setup for descending.


The high and low speed adjustments are found only in products more devoted to descent / gravity.
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 fork to move rapidly towards 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 that the fork feels counts.
FORK 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, keep the low and high speeds halfway: in this way you will have fairly neutral suspension and from there you will evaluate whether it is appropriate to open (-) or close (+) the compression. Know that a fully open fork (-) can be comfortable but less performing than a more closed fork (+), which will remain more supported on obstacles and will work using the hydraulics in the best possible way.

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 -) we will find an unstable front end that loses precision by extending too quickly (each fork dip/extension is a change in steering angle).

Rebound is a hydraulic brake that reduces fork extension speed. To adjust it, we find a "knob" located below the sheath (or above in the case of upside-down forks) 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 sudden wheel slips.
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 find the right setup, remembering again that there is no rebound value that fits everything: for example a faster rebound can be fine on technical and slow trails to avoid being catapulted into forward after the fork has gone flat, while on fast and flowing terrain a slower rebound is better to have greater stability and not be kicked on bumps/jumps... but then, to close, every rider has his preferences! The same goes for compressions.

NOTE: In the more sophisticated models, such as Ohlins, the rebound does not affect the compression due to the technology double tube which keeps the compression separate from the return (characteristic that distinguishes the Öhlins) and a revolutionary internal hydraulic system where the main difference is represented by the work of two internal chambers which, thanks to the help of two valves, manage and regulate the flow of the 'oil.

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
