What is a converter dolly, as defined in the AAMVA CDL Manual?

49 CFR 383.115(a) ("Procedures for assembly and hookup of the units"); AAMVA CDL Manual Section 7.2.1 "Coupling Twin Trailers," Figure 7.1, p. 7-2

Understanding exactly what a converter dolly is and does is the foundation for every other doubles/triples topic, from coupling procedure to the dolly-specific items checked during a walk-around inspection.