Parts of speech or participial construction (insertion of "palm face up")

People don’t usually think of touch as a temporal
phenomenon, but it is every bit as time-based as it is spatial.
You can carry out an experiment to see for yourself. Ask a
friend to cup his hand, palm face up, and close his eyes. Place
a small ordinary object in his palm ― a ring, an eraser,
anything will do ― and ask him to identify it without moving
any part of his hand.

One idea: palm face(two word noun) + being(present participle) + up => palm face (being) up

Another idea: palm(noun) + face up(adverb) => palm face up = with his palm faced up

Research

Adverb
face-up (not comparable)

In a position with the face upwards.
The cards must be dealt faceup(sic)

https://en.wiktionary.org/wiki/face-up