A few years ago I made a similar flagging deal like this for use with a pull string, our blind were in tree cover (much like duck hunting) what we learned was when the birds are close & circling low and about to commit it was best NOT to flag as it scared em off. Use a flag to get them to notice when passing over high cause once they see the spread and they are interested they will come around for a closer look but will flair if flagging is used closer in.
This has been my experience in tidewater river areas with grass fields.