These animations illustrate Cycloidal Gearing, which is more commonly associated with watch making. The first animated GIF shows a typical application, while the second one shows a rotary engine application, where one gear is fixed. The smaller gear is called the external gear, while the larger gear is called the internal gear -- it seems backwards, but the names are in reference to the tooth direction, not the relative gear locations. In a rotary engine application, the smaller, fixed external gear is appropriately called a Stationary Gear. The larger, internal gear is affixed to the Rotor and moves eccentrically. Please refer to RE101: Mechanics for a more thorough explanation.