How cylindicity works?

Greenimi
It is used on precision bearing diameters for gear shafts in my field.
It limits taper
It limits roundness
It limits straightness
But can be any specified diameter.
Post an example and I can decipher it.

The zone is adaptively cylindrical. The overall shape can be an hour-glass as long as no intermediate limited section exceeds the tolerance.

The explanation is clear.

The inability of the standard to create a proper recognition of 3rd order variations is the problem they are trying, somewhat unsuccessfully, to solve.

greenimi,
A cylindricity tolerance zone, whether per unit length or full length, is always 2 coaxial cylinders.
Suppose a measured zone that is as long as the full length of the actual cylinder is best fitted over the actual 100 mm long cylinder and it should measure up to 0.4 mm. Then a 10 mm long measured zone related to the unit basis requirement is fitted over some sampled sections along the cylinder and it can only measure up to 0.06 mm.

If you understand per-unit straightness (Fig. 8-5 from Y14.5), then cylindricity is somewhat similar: imagine a regular cylindricity zone, but instead of extending for the entire length of the feature, it is only for the designated length. The zone can slide along the feature, allowing some variation over long stretches but preventing sudden kinks within a short segment.
Of course, it's different than Fig. 8-5 because cylindricity is a surface control, but the idea of a zone that slides along might help.