A chain may use both kinds of strong link, alternating with weak ones, and at least one end is true.
Where to lookA pointer first: link the marked cells into a chain through 3 and 6.Alternating inference chainFollow the chain: either row 4, column 2 is 6 or row 7, column 2 is 3, at least one of them. Were a struck 6 true, neither end could be, so it goes.
A strong link is a cell with two candidates, or a digit with only two places in a region. A weak link joins two candidates that see each other and cannot both be true. Alternate them. At least one end must hold, so remove any candidate that contradicts both ends.
This example, step by step
Where to lookA pointer first: link the marked cells into a chain through 3 and 6.
Alternating inference chainFollow the chain: either row 4, column 2 is 6 or row 7, column 2 is 3, at least one of them. Were a struck 6 true, neither end could be, so it goes.
In hints
The hint first gives a direction. Blue marks the chain step by step, and orange strikes remove the candidates that contradict both ends.