This topic should be base for future FAQ about revegging.
Re-vegging (a.k.a rejuvenation) means switching plant back to vegetative stage after it has been flowered.
It is done by switching plant from flowering light cycle 12/12 back to vegetative cycle using 18/6 or 24/0 lighting. Plant that has been flowered is partly harvested and only few flowers are left on the plants. It is stressful practice, but useful for saving genetics or monster cropping.
First leaves that protrude from flower are usually stressed and it takes anywhere from 2 - 4 weeks.
Picture of revegged clone (see bud from which new shoots have grown)