“Turning the top” requires first having an “openish” feeling as one sings higher.
One should not try to force complex top of the range muscular adjustments too soon in the development stage. One should not try to force complex top of the range muscular adjustments too too low in the pitch range. One should not try to close or cover or darken the vowelled tone to force the voice to sing higher.
For when the vowel and shape of the mouth cavity forces the registration to work, there is unhealthiness. Beyond that, forcing and making up voice is not a fun way to sing.
It is much better to respond to an exercise designed by a knowledgeable teacher to encourage muscles to work automatically, rather than the singer striving in a futile manner to micromanage all these muscles.
One responds to the exercise given, ascending the scale. Normally, one will feel a stepping back and up with the sound and vowel being “openish” – without too much change from the bottom except for being lighter.
We focus on development of the closer and stretcher muscles; learning how to adjust the vocal cords in a healthy way. We pay attention to the feeling of the staircase from heavier, thicker, down and front to lighter, thinner, up and back. This process will give us a sound that is openish as we ascend the scale. As the voice develops, changes in vowel and the “turn” begins to happen automatically in response to healthy registrational adjustments.