From the AIX4 FORUM on IBMUNIX on 10-23-95 by Dick Dunbar,

There are two ways to enable fast boot:

Cold start:
 Local power off (i.e., just the service "bump" processor running)
 Key to System, aka Service Mode.
 Press Enter .... you get a prompt:  >
 > sbb  <enter>
     If the system just looks at you stupidly and does nothing, insure
     you have the key in Service mode.
 1    (Change flags)
 6    (fast ipl)
 y    (Change disable to enable)
 x    (Exit)
 x    (Exit)
 Switch key to Normal
 Local power on

The other way is before you bring the system down, enable fast ipl
and bump console with:
  mpcfg -cf 11 1