How to use prelim option in sqlplus

1) sqlplus -prelim / as sysdba

2) sqlplus /nolog
set _prelim on
conn / as sysdba so I tried…

-bash-3.1$ sqlplus /nolog

SQL*Plus: Release 11.1.0.6.0 – Production on Thu Sep 25 18:04:28 2008

Copyright (c) 1982, 2007, Oracle. All rights reserved.

SQL> set _prelim on
SQL> conn / as sysdba
Prelim connection established
SQL> oradebug setmypid
Statement processed.
SQL> oradebug unlimit
Statement processed.
SQL> oradebug dump hanganalyze 1
Statement processed.
SQL> oradebug dump systemstate 10
Statement processed.
SQL> oradebug tracefile_name
/oracle/admin/diag/rdbms/demo11g/demo11g/trace/demo11g_ora_25010.trc