Here are some wonderful quotes from Isaac Nathan’s “Misurgia Vocalis” published in 1836, on using the feigned voice, or what might today be considered the voce faringea or voce della strega.
I highly recommend this book to all teachers of voice as an excellent example of the teaching methods of the early 1800s.
There is a break, more or less, in the voices of both sexes, but more particularly in that of the male, between the Voce di petto and falsetto: that precise part of the vocal organ where the Voce di petto forms this juncture with the falsetto is by the Italians called Il Ponticello, “the little bridge:” and singers who can with safety carry this Voce di petto over this little bridge may truly sing its praises. It should here be an object with the Singer to contrive to blend the two qualities of tone, at their junction, in such a manner, that the transition from one to the other may not be perceptible to the ear. This cannot be accomplished without the aid of the feigned voice (ed. Pharyngeal voice), which may be justly considered the only medium or vehicle by which the falsetto can be carried into the Voce di petto.
The Tyro (student) having this far satisfied his ear in distinguishing the feigned voice from the falsetto, should endeavor to blend those two qualities of tone by commencing with his falsetto upon any given sound, and whilst in the act of prolonging that sound, change the vowel without taking a breath, as [e-a] or [u-a] this will decidedly effect the desired union; which having been accomplished, the next object must be that of uniting the feigned voice with the Voce di petto.
Let every note be begun in the feigned voice as softly as possible, by swelling gently, and immediately returning to the first piano: as the voice increases in power and quality let the swell be increased, yet with caution; the slightest irregularity or roughness being a sign that the singer has exceeded the development of which his organ is yet capable. Any unsteadiness or tremor of voice is to be remedied by taking the note softer; a contrary course only serving to increase and confirm the defect.
Nathan, Isaac. Musurgia vocalis. British Library, 1836.