Down & Front as a sensation is experienced when one sings in the lower area of the voice or as one descends the scale; assuming all is well.
The reason the singer has this sensation is directly related to the size of the glottal slit – space between the cords – which is determined by how much cord in length, depth, width is vibrating to produce the lower pitches.
Let us say that we are walking down the vocal steps from the second floor to the first floor. As we descend the scale there is more length, width, and depth of cords vibrating. Thus, the size of the glottal slit increases. As this occurs, the air column – vibrating molecules – that moves through the slit is not as crowded. This column becomes wider in diameter, shorter, and begins to angle its direction from the back to the front, from feeling like its up and back of the soft palate to being directed underneath the hard palate on the lowest notes.
Now, there are variations in this sensation given the specific vowel, but in general when we are singing in the lower full voice there is the feel that we are Down & Front; a reflection of what is happening correct at the source!