A foil shaver holds a thin perforated metal screen over oscillating cutters. Hair enters through the perforations and is cut flush at the skin surface, with the foil itself acting as the barrier between blade and face. That geometry produces the two characteristics foil shavers are bought for: the closest shave available from an electric device, and the lowest irritation for sensitive skin — because nothing twists, lifts or pulls the hair the way a rotary cutter does.
That last point is why foils are the standing recommendation for men who get ingrown hairs and razor bumps. An ingrown hair forms when a cut hair retracts below the skin line and grows back into the follicle, and lifting the hair before cutting is what causes the retraction. A foil cuts at the surface without lifting, which removes the mechanism rather than managing the symptom. On coarse, curly beard hair — where bumps are most common — this is the single most effective change you can make.
The trade-off is honest and worth knowing before you buy. A foil is a flat cutting surface, so it follows a strongly angular jaw less easily than a rotary's independently flexing circular heads. If your face is very contoured, or if you also shave your head, a rotary may genuinely suit you better. And foil cassettes are consumables — from ₹2,400 for a Braun Series 3 to ₹6,000 for a Series 9, due every 12 to 18 months.