"Nerve stroke" or "feathering." Use your finger tips (you want a small surface, not broad) and light pressure. Your gf would probably also love this scalp massager, especially if someone else uses it on her scalp, big shivers.
Scalp Massager 2-pack – Handheld Head Massage Tingler, Scratcher & Stress Reliever Tool Set for Hair Stimulation & Relaxation by Body Back Company (Colors May Vary) https://www.amazon.com/dp/B009VJS03U/ref=cm_sw_r_cp_apa_i_vu4dBbWZCFEJ0