Verb or adverb?

[SkyFox]

I watched a video and this woman explained the following word CHECKING in the sentence functions as a adverb... but I don't see how.

" The student was sitting at the back of the room checking her email"

Checking describes what the student was doing, so should it not be a verb? Or does checking describe sitting and thus is an adverb?

[Alan]

The word 'checking' itself is a nonfinite verbal form (to be precise, a participle), but the PHRASE that it introduces can be considered as having an adverbial force (i.e. = while checking....).