Rikhawah is the continued flow of sound during the articulation of a letter. It contrasts with shiddah, in which the sound is temporarily blocked completely. The relevant distinction is the behavior of sound, not recitation volume.
For example, an unvowelled سْ can be sustained momentarily, whereas قْ cannot be sustained in the same manner. Rikhawah does not permit an arbitrary extension of س, ش, or ف in recitation.
Letters outside the eight of shiddah and the five of tawassut are classified under rikhawah. The three terms together describe a spectrum of closure, intermediate restraint, and continued sound.