This term literally means "loss of muscletone" or "absence of retention" in Arabic.
Technically, Ar-Rakhwa is the flow of the voice during the pronunciation of the letter due to a weak support of its point of articulation.
Thus, for the remaining of the letters, in the absence of vowels, their articulations are made with a total flow of sound;
This is in part due to the "continuous fricative", "continuous constrictive" character of the letters.
Characteristic concerning the letters:
Tha(ث), Za (ز), Siin (س), Chiin (ش), Saad (ص), Daad (ض)
Zaa (ظ), Rain (غ), Fa (ف), Ha (ه), Waou (و)
And Ya (ي).