The patient attempts to produce sound at their lowest pitch, at any volume, low or high. This defines the vocal floor of their voice. Sometimes the person has excellent vocal rapport, capable of matching their voice to notes played on a piano. Some people are not so talented and the examiner may ask them to slide down in pitch and then by ear try to determine the lowest note produced.
Moving lower in pitch removes any compensation from the cricothyroid muscle. For example, increased air leak at lower pitches represents a gap, which could be from bowing or paresis which is often masked by cricothyroid tension at higher pitches.
I can diagram this low range in a normal individual.