re: Rules on Redshirting

In college football, a student athlete may redshirt to increase size and strength, completion of physical maturity, desirable assets for many positions. As the college years coincide with the typical sing a year of eligibility in the last college year is generally more beneficial to the team and to the student athlete's potential professional prospects than it is to use the same year of eligibility in the first college year. Players, especially in football, may redshirt to learn the team's play book, since college teams typically run a greater number of, and more complex, plays than most high school teams.
An athlete may be asked to redshirt if he or she would have little or no opportunity to play as an academic freshman. This is a common occurrence in many sports where there is already an established starter or too much depth at the position in which the freshman in question is planning to play.
A special case involves the eligibility of a player who loses the majority of a season to injury. Popularly known as a medical redshirt, a hardship waiver may be granted an athlete who appears in fewer than 30% of his or her team's competitions (with none coming after the midway point of the season), then suffers a season-ending injury. A player who is granted such a waiver is treated for the purposes of his or her eligibility as though he or she had not competed in that season.