The ENCRYPT switches located on panel O7 control the integrity of the Y-code key within each receiver. In NORM, the key is kept intact such that it can be used to decode the Y-code signals being sent by the NAVSTAR satellites. If this switch is moved to the ZEROIZE position, the key will be erased from the memory of the receiver and the receiver will only be able to make measurements using the civilian codes. This is a procedure that would be used in the event of a non-U.S. landing to protect the key from being obtained by unauthorized users. A permanent switch guard prevents accidental movement of this switch.
Source: wiktionary