Mysterious Technique (in Japanese, 謎の技術 Nazo no Gijutsu) is a tag depicting an illustration done with questionable techniques, often mysterious unless the creator or the illustrator decide to disclose the process of the work itself.
On pixiv, it is often regarded as :-
- Too extraordinary, you don't even know how it's done.
- The process itself is unimaginable.
- You don't know what tool or program is used for this illustration itself.
The origin of the tag is most likely from NicoNicoDouga.