The idea is simple: every letter has an inside and an outside–Internal contrast is when stroke emphasis is applied to the outside of the letter-shape, with the strokes on the inside of the letter-shape being the hairline. This, combined with the classic sans-serif convention of pinching at the intersections of strokes and bowls creates a texture of thicks and thins that behave functionally in much the same way as a standard high-contrast expansionist sans-serif, even though aesthetically it is wildly different.