An e-file is the electric rotary tool — essentially a precision drill with interchangeable bits — that techs use to remove old product, prep regrowth at a refill, refine enhancement surfaces, and (with fine bits) lift and clean the cuticle area. In skilled hands it is faster and often gentler than hand-filing, because an experienced tech removes product without ever touching the natural nail plate.
The skill gap is the whole story. An e-file should never hurt: heat spikes and stinging mean too much pressure or too coarse a bit, and a professional adjusts instantly. Filing that routinely burns, or leaves rings of thinness near the cuticle, is a red flag worth acting on — the tool is safe; careless hands are not.