What i call a weak chain now, is a generalization of both Nick70's turbot fish and simple colouring, in the sense that the technique solves all what both can solve (but of course its not that "user friendly").
In another thread i already posted that what turbot fish can solve, can be reduced to what a chain A-B~C-D for one colour can solve, where A-B and C-D are conjugated pairs (strong sides) and B~C is a weak (or like) link (that meens, they are in the same row/col/box). If you have this chain, you can eliminate the colour from all "intersections" of A and D (cells which have both a common row/col/box with A and a common row/col/box with D, i.e. have a weak link to both A and D), because either A and D must have the color (if not, both B and C would have, but they share a row/col/box).
The generalization is simply that you can build longer chains with alternate strong and weak links and can eliminate the colours that intersect the first and last cell. It's easy to see that this holds: If you add 2 nodes for a chain A-B~C-D~E-F and F would not have the colour, then E must have it and not D, which is the situation above.
Obviously this solves turbot fish patterns, because A-B~C-D is a weak chain. It solves colouring patterns, because if you have a "strong" chain A1 to An and can eliminate a colour, because a cell is intersecting Ai and Aj, then Ai through Aj also is a weak chain.
So far i dont have a sample, that is not solvable with other techniques, but i think the following puzzle is interesting for its own:
39.|..4|7..
...|..8|...
.5.|.9.|.2.
-----------
.12|75.|.9.
7..|.1.|5..
9..|..2|6..
-----------
...|6..|8..
...|.8.|.51
..4|..5|..3
With a turbot fish in 1 you can put 3 in (3,4) and you come to here:
3___9__16____12__26_4____7__8_5_
2___4__67____5___67_8____19_3_69
18__5__678___3___9__17___14_2_46
4___1__2_____7___5__6____3__9_8_
7___6__38____89__1__39___5__4_2_
9___38_5_____48__34_2____6__1_7_
5___2__139___6___34_139__8__7_49
6___37_39____24__8__379__24_5_1_
18__78_4_____129_27_5____29_6_3_
You can solve it now with the xy wing in (2,7),(2,9),(3,7),(3,9)
The solution with a weak chain in 1 needs a long way:
(2,7)-(3,7)~(3,6)-(7,6)~(7,3)-(1,3)~(1,4)-(9,4)~(9,1)-(3,1)
(only the second side is really weak)
So you can eliminate 1 from (3,7)