What's the proposed mechanism of action of Trigeminal Nerve Stimulation (TNS) for ADHD? (And is there some neuroimaging in support of it?)

Last year the FDA approved the first medical device for ADHD, a Trigeminal Nerve Stimulator, a sort "pacemaker" that is used every night on the head of the patient.

I’m trying find what’s the proposed mechanism of action for this and if e.g. have there been imaging studies confirming how it might work in treating ADHD. Note that I’m not asking for "black box" efficacy studies for TNS on ADHD; I’m sure those exist as a precondition for FDA approval–the link above actually discusses those clinical trials in fair detail. But says just this about mechanism and neuroimaging:

While the exact mechanism of eTNS is not yet known, neuroimaging studies have shown that eTNS increases activity in the brain regions that are known to be important in regulating attention, emotion and behavior.

I can’t seem to find those studies… probably searching for the wrong term(s). I doesn’t help that TNS has been proposed for migraine, epilepsy, TBI recovery etc. as well.