When the tip of the tongue rests on the roof of the mouth near the hard palette, the air passage is open and unrestricted, FWIW.
. . .. I actually never noticed that before. I mean, even with the lips closed, if you self test how much air can come down your windpipe from your nose with and then without the tongue up, you can really fel the difference. For some weird reason, I even feel reverse breathing is harder if I don’t put the tongue up. I just knew the rule but never noticed the mechanism untill just now.