Gracias @schmitt trigger & @dendadAnother way to look at it is to just use 2 diodes for a start, like the 2 right hand ones. Have the center tap at 0V.
Now you will get around +12V, that is 9VAC x 1.414.
Next, connect it up using only the left hand pair of diodes, still CT at. 0V.
The output now is -12V.
Now, with all 4 diodes connected.......
So in effect, you have a dual power supply, +12V, 0V, -12V.
But if you take the CT off the 0V connection and put the -12V one there, you end up with +24V, +12V, 0V.
The voltages are the same, just the reference point has changed.
Now it is clear. So evident now...
I simulated to play a little. Values are made just to get some results.
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