Retuning vowels using fully interactive IPA Help which is available to PC users.
The Dropbox screen shot mentioned below shows the relevant features so that Apple users can see what they're missing. As you can see by the "red" button at the top, the user is able to sing and record the vowel and then compare it with the vowel that is highlighted, such as the "ah" open vowel is at the lower right corner. Using the form to the right of that highlighted vowel provides a demonstration of the darker form which I understand results in the tuning of the third harmonic to formant two.
It should be mentioned that these are illustrations of speech vowels and that singers need to modify the vowels they use such as to accommodate the demands of formant resonator space derivation so they match the harmonics of the pitch they are singing.
The formant spike we hear is as a result of the harmonic matching some part of the bandwidth of the formant space we have contrived and it is this space we manipulate with vowel modification.