Drago is right... but there is more.. if you go to page 11.133 of the shop manual it goes on to say you may need to do it if you have installed a new part (Like an exhaust system) that has different characteristics than the old part.
The engine management system is adaptive, and should operate from a fixed base line within a ceratin parameter. If you go outside these parameters... like moving from Vail, Colorado to Miami Beach, you may need to reset the base... ie do the 12 minute tune. If you mess with certain sensors (and I'm not going to tell you which ones) you will need to do the 12 minute tune. The 12 minute tune forces the ecm to make adaptive changes.
As Disco so perfectly stated... if you use the kill switch the ecm may loose its ability to "remember" its last settings, but that will sort itself out if you cease to use the kill switch after 3 running cycles. This is because you really haven't changed the baseline settings. It will "find its way" so to speak. I advised pbbeck to try the 12 minute tune soley because it is about the only thing he can do at home without test equipment... and he really can't hurt anything by trying it. The only way this will fix his problem is if the problem was caused by a sensor wiring harness having an intermitant open circuit which may cause the ecm to think that the baseline had changed. You can have an open circuit without throwing a check engine light if that open circuit is brief and isn't duplicated by the ecm.
Anyway... everyone is right.. I think.. :-D :-D except for me who is always wrong...
But you shouldn't do the 12 minute tune just because you feel like it or you think you might get 1.2 extra horsepower out of it. As soon as you do it the ecm starts to "adapt" itself right back to where you were originally...
:-D whew :-D