Firstly, all the voices without exception show remarkable flexibility, the tone is made to come and go in the form of sudden or gradual messa di voce in a way which greatly emphasizes both the dynamics and the varying emotional content of the music: and the melismatic passages are executed with positively startling finesse and ease.
Secondly, the tone of all the singers is of rock-like steadiness, giving a marvellously continuous and unbroken flow from note to note of absolutely uniform volume where expression does not require any change. In other words they all possess a superb cantilena, a quality which is extremely rare in modern singers, and which is not appreciated by audiences and conductors whose standards have been conditioned by singers who are inferior in this regard. The use of bravura and portamento in achieving this cantilena is made with marvellous delicacy and precision, and without necessarily involving any change of tempo or loss of rhythm. (Conductors should note that this is possible.)
Thirdly, and most striking of all, the rock-like steadiness of the tone is shot through with an astonishing vibrance, or, to use the Italian term, vibrazione. The result is extreme vitality of tine, a passionate intensity of vocal utterance which is entirely independent of both the marks of expression in the music, and of the emotion engendered in the singer by the music. It goes on all the time and is a quality of tone which works emotion in the listener and which arises purely from the physical process of phonation. This extreme intensity of tone, this emotional-tone, is the main character of the Bel Canto singer.
What all this adds up to is, of course, that those singers sang in a way which allowed of the maximum possible freedom of physical function of the voice. This was the concealed athletic art which revealed their aesthetic art, and which gives the listener an impression of utter simplicity and naturalness in their singing, quite different from the feeling of fuss and elaboration conveyed by the modern singer. It conveys also an impression of concentration and great conviction.
Kay, Elster. Bel Canto and the Sixth Sense: A discursive and technical essay on the traditional Italian physical style of singing examined by means of the facilities of modern science. London: Dobson Books, 1963.