That is a user preference... one way to "change it" is
To turn it on:mdl keyin calculat calc userPrefsP->smartGeomFlags.optimizedFenceClip=1
To turn it off: mdl keyin calculat calc userPrefsP->smartGeomFlags.optimizedFenceClip=0
To toggle it: mdl keyin calculat calc userPrefsP->smartGeomFlags.optimizedFenceClip=!userPrefsP->smartGeomFlags.optimizedFenceClip
(careful with word-wrapping on the last one... that is all one statement)
That said, given some of your previous inquiries, that may not be what you are looking for, so could you clearly identify what it is you want to achieve? Maybe there is another way to accomplish your goal.