This would give neurons surrounding the supposedly inert area the ability to grow into that area and become associated with it.(Shreeve 1993) (Ramachandran 1992, 1995) 
Ramachandran came up with idea of hiden circuitry in the brain as an explanation for phantom pain. He did an experiment using Q-Tips where he found that regions of an amputated hand are mapped out on diferent areas of the body. Ramachandran would lightly brush the left side of a amputee's chin with a Q-tip, and the amputee would remark that the brush caused senations to occur in specific areas of the phantom limb. If you would like to read more on this subject, check out 
He does not think that the brain sprouts new neurons, because the remapping can occur in times less that a month or two. Less than two months is too small an amout on time for neurons to have grown and reconnected. Ramachandran believes that there already exists the neural circuitry associated with cortical remapping. An amputation stops activity in that area of the homunculus, which allows hidden neural circuits (laying in wait for this oppurtunity) to become activated and link other areas of the body through that supposedly inert section of the humunculus.(Shreeve 1993)