A stroke happens when a blood vessel bursts in the brain. The free-radicals from blood and the lack of oxygen causes brain cell death. The brain can repair the damage by rerouting the signals going to the effected brain cells to other brain cells. At least in the case of this study, the brain cells started doing double duty instead of changing jobs. Further study has to be done to confirm this.
http://scienceblogs.com/neurophilosophy/2008/07/researchers_watch_brain_rewire.php