To the beginning singer the main bridge may feel like a nasty point in the range. To the advanced singer, the 50-50 split resonance is in fact 'Home'... The issues surrounding the bridge are overcome and from a narrow point of awkwardness it becomes an octave-wide zone where most of the singing is done. We call this middle voice.
Most people either 'oversing' or 'undersing' at first. Oversingers tend to bulldose right through the first bridge, holding on to the lower voice and not allowing any change, which leaves them stuck against the barrier of second bridge. They wind up the air pressure like crazy, resulting in the poor little cords needing to call on the surrounding throat muscles to help them resist the gale of air. So the throat cramps down, the larynx is blown upwards, the singer lifts their chin and reaches up for the high notes... oh dear, something has to give... and it's then that the cords simply give up and part leading to a sudden rush of air beingh released from below the larynx. Once theis happens, it is practically impossible to get the cords back together, and the singer ends up on the higher notes in a kind of breathy falsetto. The chest voice is shut down because the larynx is so high there is no resonating cavity left, and the cords are feebly vibrating in the air-stream.
For women, this would be around the Ab above middle C. For the male voice this is the Eb above middle C. At this point a strong sense of strain has usually set in and the singer can go no higher without the voice breaking and weakening. Most people experiencing this believe that where they are stuck is where their issue lies. They think of that sticking point as 'The Bridge' and want to get 'stronger over the bridge'. We call this 'Pulling'... namely the singer is pulling the voice upwards. What they don't realise is that the problem lies far lower than that, about a fifth down, which is where their first bridge is located. Ab3 for the fellas, and Eb4 for the women. In actual fact, if you are working exercise patterns and starting at the bottom, the problem starts right at the bottom.
'Undersingers' sing with too little energy in their voices. As pitch goes up, they have no muscle in the throat to anchor the larynx down, and they don't want to feel any strain so they simply allow the cords to part (if they were ever together) and sing with a breathy tone throughout their range.