Well, welcome, my dear zjemi, but I can't help you with your question. Is this flight by birds? Insects? Or does it matter? I've never heard of this, and in fact just looking at your subject title, I thought you'd made a typo.
It sounds like it might possibly be related to nap, as in fabric. I am going to have to have somebody give me a personal demonstration of what that is, I think; despite of troy's and others' good explanations, I still don't "get it", though I know it's to do with the way the threads run.
(All I can ever see is that some go up and down, and others go back and forth.) Perhaps Helen can "see" how this type of flight might show or be related to, the "nap" of the earth.