Vertical Centering - how does it work?

[maxhifi]

Being a bit annoyed by the slight difference in the size of the black bars at the top and bottom of the screen on my CTC-38, caused me to be interested to find an answer to a question. How does a vertical centering control actually work?

Looking at various schematics of TV sets which have it doesn't seem to bring me closer to the answer.

The later "roundie" TV sets apparently have an additional winding on the vertical output transformer, which isn't really described very well on schematics. If it is out of phase, it isn't shown.. if it's in phase, then what the heck is it doing? The RCA 630 has a voltage divider which appears to be adding a little bit of signal from the horizontal circuit, again, making no sense to me. And the CT-100 and CTC-4, have a "vertical centering choke", which appears to be adding a little bit of phase shift into the waveform hitting the vertical deflection coil.

Am I missing something obvious here? I also noticed that factory service manuals which go out of their way to describe all other parts of the circuit don't touch on "theory of operation" for the vertical centering control. Also, I am getting the impression that attempting to add a centering control to the CTC-38 would entirely screw up the pincushion correction circuit.