A spiral-profiled cam used in striking mechanisms like minute repeaters. Its stepped radius determines how many times a hammer strikes to indicate the hour or quarter.
A snail cam, sometimes called a limaçon (French for snail), is a cam machined with a spiral profile that increases in radius across a series of steps, one step per unit being counted. In a striking mechanism like a minute repeater, a spring-loaded lever or rack falls against the cam at a point determined by the current time; because the spiral's radius at that point sets how far the rack can fall, it directly determines how many times the hammer strikes.
Repeaters typically use two snail cams working together, one for hours and one for quarters, each geared to the relevant part of the going train so its profile always corresponds to the correct count at any given moment. Worth noting this is a different component entirely from the heart-shaped 'coeur' cam used to reset a chronograph hand to zero, and unrelated to snailing, the decorative circular-spiral finishing technique, despite the shared spiral imagery in both names.
Its spiral profile increases in radius across steps, one per count. A lever or rack falls against the cam at the point corresponding to the current time, and the distance it falls, set by the cam's radius there, determines how many times the hammer strikes.
The name comes from its spiral shape, which resembles a snail's shell. It's also commonly called a limaçon, the French word for snail, in watchmaking literature.
Typically two, one for hours and one for quarters, each geared to the relevant part of the movement so its profile always reflects the correct count at any given moment.