Pharyngeal Falsetto is a falsetto voice developed using certain vowels that help us "find" this specific feel and hear this specific sound.
This sound can be nasty or annoying. Yet, this voice is essential for a male to sing the top of their range, and the female to sing the middle of their range. This type of falsetto sound exists because the crico-thyroid muscle system is properly exercised and developed without constriction, as well as properly coordinated in a healthy relationship with the chest voice.
If we looked in someone's larynx and analyzed the activity of the cords as they sang "pharyngeal," then we would see that as the crico-thyroid muscle was stronger and more active, there is greater closing activity in the arytenoid muscle system, and there is greater depth of vibrating cords.
Note that pure falsetto engages the least cord in depth, thus there is shallow vibration. Whereas, whenever one is in full voice, there is always top and middle, and at least some of the bottom layer of the cords vibrating to create the full voice texture. The pharyngeal falsetto engages the top and middle layers of the cords only. Thus, it acts as a “bridge” between falsetto and full voice!