What is Backing?

Legal Definition
Crim. law practice. Backing a warrant occurs whenever it becomes necessary to execute it out of the jurisdiction of the magistrate who granted it; as when an offender escapes out of the county in which he committed the offence with which he is charged, into another county. In such a case, a magistrate of the county in which the offender may, be found, endorses, or writes his name on the back of the warrant, and thereby gives authority to execute it within his jurisdiction. This is called backing the warrant. This may be from county to county, if necessary.
-- Bouviers Law Dictionary
Legal Definition
Endorsement; endorsement by a magistrate.
-- Black's Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary