* If they've already redshirted: A player who has already used their redshirt season can play without restriction for the remaining years of their eligibility. The redshirt year is in the past; their eligibility clock is simply running.
* If they haven't used their redshirt: Playing in a game burns the redshirt season. They've used a year of eligibility, and they cannot retain that year. They will then have the standard number of years of eligibility remaining (typically four) minus the one year used.
In short, the key is whether the player *has already* redshirted. Redshirting is about *preserving* a year of eligibility; once that year is preserved, it's not relevant anymore. It's only the act of *playing* in a game while still eligible for a redshirt that forfeits that year.