Usually these residencies last one or two years less than the total years that would be spent doing both residencies. For a few specialties, a year of post-gradute training is required before beginning a residency in that field. Many who want to go into these fields fill that requirement with a transitional year. Fields that require a year before beginning residency include radiology, neurology, anesthesiology, and ophthalmology. Most specialties do not require a transitional year, but instead accept medical school graduates straight out of medical school.
Preliminary years come in two flavors, internal medicine and surgery.
Each of these preliminary years somewhat resembles the rotating internships of old, but with a focus on either internal medicine or surgery. Those programs that require a year of post-graduate education before beginning residency may accept either a transitional year or a preliminary year. Obviously, surgical residencies will require that you do a preliminary surgery year while some other specialties will prefer a preliminary medicine year. The other reason that a new M.D. would go into a preliminary year or transitional year would be because he didn't match into the specialty of his choice. The hopeful applicant then takes a preliminary or transitional year in the hopes of improving his chances and qualifications for the next year's residency match.