What makes this effective is the tagging (when done - it's not always done, and that could be a problem) - apparently the tagging process involves the stalker being court-ordered to wear a tamper-resistent ankle bracelet, a little like the ones people have to wear when under house arrest, but with a much farther reach: instead of being wirelessly tethered to a radio receiver in their home, the stalker tags upload GPS co-ordinates continuously, from wherever the wearer is.