Passaggio

Register transition (passaggio) is physiological: the larynx muscles change their behaviour when you go from low to high notes. In normal conditions the larynx tends to rise, the sound becomes lighter, forced and finally breaks.

To avoid this keep your larynx and diaphragm down, so timbre and colour will remain the same. Lighten and transform phonetics (see: phonetics modification), so the sound will move to head register without straining or breaking

See the scheme A to identify the point of passaggio according to the type of voice.