What is the tiller, and how does it work ?

It's often mentioned, and people wonder what it is for and how to adjust it: 
what is the tiller, and how does it work ?

Adjusting the tiller allows both limbs of the bow to work in the same way. Shooting with one finger above the nock and two fingers below for a recurve or three fingers below for a barebow means that the upper limb is less stressed than the lower limb. Therefore, it is necessary to synchronize the entire system to evenly distribute the energy from both limbs.

How to Adjust the Tiller ?

After measuring the distance from the top tiller to the string, measure the distance from the bottom tiller to the string as shown in the photo below. The difference in millimeters between the top and bottom should typically be between 0 and +6mm.

Bonus:

To increase the tiller, tighten the bottom screw, and to decrease it, tighten the top screw. Additional tests can be performed, such as slowly drawing the bow at about twenty meters facing a target: is your aim going up? Decrease the tiller by tightening the top screw (or vice versa if your aim is going down).
Are you on target? Congratulations, your tiller is set!