It becomes a bit confusing with all the double negatives!
If something is dangerous, then it is not "incidental"
If something affects the play, then it is not "incidental".
For example, if I bump a receiver as they are about to catch the disc, in a non-dangerous way, that contact affects the play and is therefore not "incidental" and is therefore a foul.
I had a player argue that for contact to be non incidental the contact has to be dangerous AND affect the play.
This statement above is incorrect.
Please note that for the 2021-2024 version of the rules the term "incidental contact" has been changed to "minor contact" and has a different definition:
Contact that involves minimal physical force and does not alter the movements or position of another player.
Contact with an opponent’s extended arms or hands that are about to, or already are, contacting the disc, or contact to the throwers hand during the throwing motion, is not considered to be minor contact.