What happens above the open mouth AH position? As the singer opens the mouth wider with the rising pitch, a switch from full vocal track resonance, (which the singer was doing up to “the break”) into a new adjustment, mixing the harmonics and switching to mouth resonance alone, creating a new tube or container with a different size, and thus, a different resonant pitch than was had with the full vocal tract. (closing off the nasal passages and tapping on the cheek the same way the throat was tapped to get the full vocal tract resonance, we now get a resonant pitch approximately a fourth above the full vocal tract resonance. This causes overtones of one (the full vocal tract) and two (the mouth resonance) to switch, causing the effect many people call "covering" to occur. Look at it this way: if F4 is wide open mouthed, the same F4 in mouth resonance alone becomes it the OH shape of middle C (C4). Try it out! Sing a nice F4 (if you're a guy - if a gal, it would be F5) Find the resonant position by doing the tapping exercise - mouth in AH position: vocal cords closed: tap on larynx: then sing the pitch you hear when tapping (an octave higher for ladies). Your mouth should have been open wide (just a little wide). Now find the same pitch you tapped and sang, but now tap on your cheek (having closed off your nasal passages by raising and locking the soft palate. In order to get the same pitch, you had to make an OO shape! Now sing the F4 with the new OO shape. It might be easier, and it will sound "darker" or richer.
  Had this switch been some other interval I would never have discovered this trick of the passaggio - an harmonic switch which corresponds with the harmonic series, and gives the tone the rounder sound we expect when a hearing a classical singer.