All chickens have two copies of every gene, one on each chromosome of a chromosome pair. EXCEPT for the sex chromosome pair.
Males have two Z chromosomes; females have one Z chromosome and one W chromosome. The Z chromosome is longer than the W chromosome, and thus contains genes that are not on the W. Think of it like this:
Z --------------------------------------------
W ---------------------------------
The genes that are on Z but not on W are the sex-linked genes.
Theoretically, any sex-linked gene could be used to create sex-linked offspring, but many are impractical because the trait is not apparent or doesn't develop until the bird is older.
The two genes most commonly used to create sex-links are silver and barring. If you breed a hen carrying the dominant allele to a cock carrying two copies of the recessive allele the offspring will be visually different at hatching.
With silver, S- + ss gives Ss sons and s- daughters. With barring, B- + bb gives Bb sons and b- daughters.
If you take the sex-linked males and try to re-create the link, you would have
Ss + S- and have SS, Ss, S- & s- offspring
or
Bb + B- and have BB, Bb, B- & b- offspring
When a hen creates an egg, she gives it a single set of chromosomes built from all the genes she carries. If she gives it a Z chromosome, the egg could develop into a son; if she gives it a W chromosome, the egg could develop into a daughter. When a cock fertilizes the egg, he gives it a single set of chromosomes built from all the genes he carries. He always supplies a Z chromosome.