Chromosome walking is the technique for cloning everything in genome around the known piece of DNA (the begning probe). You screen the genomic library for all the clones hybridizing with probe, and then figure out which one extends furthest into surrounding DNA. The most distal segment of this most distal clone is then taken in use as a probe, so that ever more distal areas can be cloned. This has been taken in use to move as much as 200 kb away from the given begning point (an immense undertaking). In general it is used to "walk" from a starting point towards some nearby gene in order to clone that specific gene. It is also used to obtain the remainder of a gene when you have isolated a part of it.