If we assume that I5 contains the value 8, it follows (through a Forcing X-Chain) that I5 cannot contain the value 8. Therefore we can conclude that I5 does not contain the value 8.
Unary Chains start with a simple assumption that a cell is/not one of its potential values, which is proven to be either true or false.
X-Chaining means that:
(1) when a cell is assumed to be a value
then other possible positions for this value in this cells three region are
removed
(2) when a cell is assumed to not be one of its potential values
and there are only two positions for this value in one of this cells three
regions
then the other cell must be this value.
The forcing chain consists of a chain of implications based on simple rules. The chain is an X-Chain, because it only involves a single value. The details of the Forcing Chain are given below.
Note that Unary X Chains which contain 6 cells are also known as Turbot Fish.
Details of the Forcing Chain:
(1) If I5 contains the value 8 then G5 cannot contain the value 8 (the value can occur only once in the box)
(2) If G5 does not contain the value 8 then G9 must contain the value 8 (only remaining position in the column)
(3) If G9 contains the value 8 then E9 cannot contain the value 8 (the value can occur only once in the row)
(4) If E9 does not contain the value 8 then E4 must contain the value 8 (only remaining position in the column)
(5) If E4 contains the value 8 then D6 cannot contain the value 8 (the value can occur only once in the box)
(6) If D6 does not contain the value 8 then B6 must contain the value 8 (only remaining position in the row)
(7) If B6 contains the value 8 then B1 cannot contain the value 8 (the value can occur only once in the col)
(8) If B1 does not contain the value 8 then C1 must contain the value 8 (only remaining position in the box)
(9) If C1 contains the value 8 then C7 cannot contain the value 8 (the value can occur only once in the col)
(10) If C7 does not contain the value 8 then I7 must contain the value 8 (only remaining position in the row)
(11) If I7 contains the value 8 then I5 cannot contain the value 8 (the value can occur only once in the col)