One of the best things a voice teacher can do is to constantly ask the question, “Is the voice I am listening to right now too heavy or too light?” In mix we are trying to find a good balance between “hanging on” and “letting go”. There needs to be ‘release’ into the head voice, but there also needs to be ‘cord closure’ hanging on to the chest voice as well. As you listen to the student, if you ask yourself the question, then answer it you can then better choose an appropriate exercise to get the student closer to where they should be. If they are “too heavy” use ‘release’ exercises like octave down with a narrow vowel. If they are “too light” use “bottom up” exercises with more open vowels with hard or medium consonants to hold on to the vocal cords better.
Many times it is only a subtle adjustment that is needed so simply moving to a harder consonant or to a slightly more open vowel will move the “light voice” slightly “heavier”. On the other hand an aspirate consonant (like ‘f ‘or ‘sh’) or using a slightly more narrow vowel may all that is needed to get the voice that is a “too heavy” a little “lighter”. Exercises from the “top down” will also “lighten” a voice.