The 3D Medusa technique can be seen as an extension of the Simple Coloring technique, where we now also consider different candidates (not only one digit).
If you don't know about Simple Coloring yet, take a step back and give it a read.
How does it work?
For Simple Coloring we had a chain of strong links where we could say: "Either all candidates are correct, or all candidates are correct."
Every candidate that sees (weakly links to) both colors can be eliminated.
Simple Coloring is reduced to one specific digit, 4 in this case.
For 3D Medusa we do exactly the same, but we don't just consider links between cell candidates within a region. We now also consider links between different candidates (digits) within a cell.
Strong and weak links can be easily applied to the different candidates of one cell.
If there are only two candidates within a cell, then they are strongly linked.
If there are three or more candidates within a cell, then they are weakly linked.
Except for those additional links, the logical train of thought is still the same as with Simple Coloring:
If 7 is the solution,
- then 2 is not,
- then 2 is,
- then 3 is not,
- then 3 is,
- then 7 is not.
7 is weakly linked to both colors, so we know that 7 cannot be true, so we can eliminate it.