Overview:
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GW interaction
Polarity symmetry break
Hone GW values
Use V leverage on the GW

GW Interaction
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Colour for polarity with 4 colours.
If you pair corner with edge you can't colour the outside regions.
=> The corners are the same colour/polarity.

Polarity symmetry break
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By definition V puts 1234 on the two cells.
Along the modular line, it can't be 1-4, so it's 2-3
Extend polarity colouring into r[25]c[25]
Pencil mark [123]s and [456]s
At this point, you can delete the colouring, it's done its job.

Hone GW values
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Third cell on the modular line is [14], but is same polarity as [23], so is 1
=> r6c5 is 1 also
Remove 1 from cells that see the known 1s
Remove 4 from the cells neighbouring [23]s
=> The [56] pair in the row means r2c2 is 4, ditto column 5 and r5c5.

Use V leverage on the GW
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Notice how all the 5s are on one GW, and all the 6s are on the other
r1c5 can't be 3, as that pushes 6s onto its GW, and 5s onto the other, however, it also pushes a 3 onto the other GW, breaking it.
=> r1c5 is 2, r2c5 is 3
=> 2s windmill around the grid
=> row 2's modular line resolves, which resolves the whole row

Bring it home
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The 5 in r6c1 means that r5c1 can only be 1
=> everything resolves apart from [34]s round the outside
The final two modular lines find the 3 and the 4 that complete the puzzle.