At Alabama, if a player has an injury history or is "underperforming" they are informed that their chance of seeing playing time is close to zero.
Those that want to keep playing will transfer. Some that want to stay in school will be offered other aid so that they can complete their studies... they just won't be part of the program nor count against their scholarship limit.
For the incoming class, some will be asked to defer their enrollment. They're not informed of this change until just before signing day, which makes it nearly impossible to choose another school.
The SEC has recently (2012) attempted to reduce the over signing practice, but there's still some going on.
Here were the worst offenders a couple of years ago: