His example builds the class off of a set of test for a "BowlingGame" that ends up with roll() and score() methods based off of the naming conventions used in the test:

Following the convention that the tests for a class BowlingGame (see below) are written in a class named BowlingGameTest (see above), the test case class' source is searched for variables that reference objects of the BowlingGame class and analyzing what methods are called on these objects.