How does an entry-level contract slide?
If a player aged 18 or 19 signs an entry-level contract with a club (with his age calculated on Sept. 15 of the year he signed the contract) but does not play in at least 10 NHL games — regular season or playoffs — the contract will "slide" or be extended one year. The extension does not apply if the player turns 20 between Sept. 16 and Dec. 31 in the year he signed the contract.
Depending on the contract's structure, the player's cap hit can be affected either by an increase or a decrease. Players who sign at age 18 can have their contract extended (or "slide") two seasons.
CBA reference: Section 9.1 (d) (P. 23-24)