After planking, owling and leisure dive, now there is the trend which is called "Batmanning". Obviously it is a made-up word. Is there a proper substitute for it? What is the verb that defines the act when a bat hangs from somewhere?
"Planking" is just as incorrect as "Batmanning" other than the spellchecker is okay with it for unrelated reasons. If the masses of the internet feel like coining a term that compares 'hanging like a bat' to 'planking' then so be it. The same goes for "owling".
Assuming you meant a word that just means, "hanging upside down":
There isn't usually a way to distinguish "upside down" from "upright" positions with these but context generally helps:
He hung from the rafters.
Teenage trends are all about being different from the oldies, and I am sure the 'Batmanners' would be mortified to find that there already was a word for the activity they have invented. That said, I think they are fairly safe in assuming "hanging upside down like a bat while somebody takes a picture of you" is not popular enough to have a single-word synonym.
You might describe it as resupinate. Although resupination means to appear upside-down, I think in a human context, the hanging part would be implied.