Question on OPM as 0

Hi here

I went through PDF 1.7 reference and Pitstop Pro to understand  the difference between OPM 0 and OPM 1.

I have clearly understanding on none-zero overprint mode now.

However,I got questions on OPM 0.

1)If the default OPM from Illustrator is OPM 1,and there is also "knockout" definition when the overprint checkbox is not selected ,how could I generate OPM 0 definition from Illustrator or other creative application? So far I could just change the definition in PitStop Pro.

2)What's the result difference between the definition of OPM 0 and knockout?

Looks like they are  the same, when viewing the different objects as OPM 0 and knockout in Acrobat Pro and also in creative application,with overprint viewing is selected.

Many thanks

Kevin