I am actually on the same wavelength with Andykhang. It is kind of Avalon from Fate series, except I don't think that it means she float to another universe or a higher dimension. It is more like she float into void/nothingness/yog-sothoth sleeping bed, etc.
Zun said it is the ultimate mystery, when Reimu use this, no one can ever beat her. (It means not Yorihime and Toyohime the Mary-Sue Lunarians, not even Yukari who can manipulates boundary or even Dragon god.)
She doesn't need to shield herself since she float away from beyond the concept of Reality, law of physics, multiverse or dimension.
Taken from Imperishable Night Last Words:
Þë▓ÒÇàÒü¿þ®ÂµÑÁÕÑÑþ¥®ÒÇéÚ£èÕñóÒü»ÒüéÒéèÒü¿ÒüéÒéëÒéåÒéïÒééÒü«ÒüïÒéëÕ«ÖÒü½µÁ«ÒüìþäíµòÁÒü¿Òü¬ÒéïÒÇé
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Supreme Master-Arts by any means. With Reimu's ability to float, she floats from everything in reality and becomes invincible.
If it wasn't just for play (with time limit), no one could beat her with any method.
Taken from various e-mailed ZUN answers from the various guy who e-mailed him:
Õ¢╝ÕÑ│Òü«ÒÇîþ®║ÒéÆÚúøÒüÂþ¿ïÕ║ªÒü«Þâ¢ÕèøÒÇìÒü»µûçÕ¡ùÚÇÜÒéèÒü«ÚúøÞíîÞâ¢ÕèøÒü«õ╗ûÒü½ÒÇîõ¢òÞÇàÒü½Òééþ©øÒéëÒéîÒü¬ÒüäÒÇìÒü¿ÒüäÒüåµäÅÕæ│ÒééÕɽÒéÇÒÇé
The ability to fly means "not be tied to anything" in addition to the literal flight capability. This means that gravity, pressure, force, etc has no meaning to her.
þø©µëïÒüîÒü®ÒéôÒü¬Òü½Õ╝ÀÕñºÒüáÒü¿ÒüùÒüªÒééÒÇüÕ¢╝ÕÑ│Òü«ÕëìÒüºÒü»µäÅÕæ│ÒéÆÒü¬ÒüòÒü¬ÒüäÒÇé
No matter how powerful her opponents is, it does not matter to her. (Something along that with Bing translator)
Òü¥Òüƒ『ÚªÖÚ£ûÕáé』ÒüºÒü»ÒÇîþë®Òü«õ¥íÕÇñÒü¿ÒüäÒü嵪éÕ┐ÁÒüîþäíÒüäÒÇìÒü¿Þ®òÒüòÒéîÒüªÒüäÒéï
Kourindo said anything have no concept of value to her. (in this state, or something along that with Bing translator)
Òü¥ÒüƒÒÇüõ©Çþ¿«Òü«þó║þÄçµôìõ¢£ÒééÞ║½Òü½ÒüñÒüæÒüªÒüèÒéèÒÇüÞ│¢Òü«þø«ÒéÆÕñëÒüêÒéïþ¡ëÒü«þó║þÄçµôìõ¢£þ¡ëÒéÆÞíîÒüúÒüªÒüäÒéï´╝êÒüôÒéîÒü»Þç¬ÕêåÒü«µäÅÕ┐ùÒüºµôìõ¢£ÕÅ»Þâ¢)ÒÇé
Reimu herself is not well aware of this power (it is the same with moving her arms or legs). Also when she uses this, she is able to knowingly manipulate probabilty (such as changing the dice roll, etc)
Yeah, this ability is still undeveloped well too because it is a mysterious ability and Reimu lacks training.
In conclusion of these whole evindence, I would agree with Promestein. She already explained about how it lacks of any feats to destroy timeline, dimension etc and it doesn't translate to her AP and dura (yet, since Touhou is not about powerlevel, but about a bunch of girls that play danmaku and drink tea together) but more about range. Also the tiering system for 1-A is clear enough in this site which stated:
"Also, mere capability to exist in a beyond dimensional domain does not qualify a character as a beyond dimensional being."
I think if Reimu uses this ability, only 1-A and above can defeat her while Reimu still retains most of her stats beside range. Last things, ZUN said PC-98 Reimu is the same person as Windows Reimu.