Ram air, a non-mechanical form of forced induction, is a feature on some vehicle bodies that lets outside air force its way into the air intake system. This is achieved by placing an opening somewhere on the body that allows air to flow in naturally as the vehicle moves forward. The faster the vehicle moves in relation to the air, the greater the overall effect.
Banks Ram Air Development
The effects of ram air are similar to that of a small turbo or supercharger, though it's not directly powered by the exhaust or engine, and the effect depends entirely on the vehicle's speed, the size of the opening allowing air to come in, and the wind direction. A small number of today's vehicles come stock with this feature, usually American sports cars and European exotics. Ram air can be added to a vehicle that didn't come with it, but it usually involves a lot of custom design work. Not all functioning air inlets on the hood are ram air, either; some are used to cool intercoolers, a technique Subaru used often in the 1990s.
Is Ram Air Effective?
As mentioned in the video by Andy's Motorcycle Obsessions, ram air works to some degree, but you definitely couldn't count on it for large power gains, especially at public road speed limit levels.