If we assume that A6 contains the value 8, it follows, through an XY Forcing Chain, that A6 cannot contain the value 8. Therefore we can conclude that A6 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
must be 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 (or more) of this
cells three regions
then the other cell(s) must be this value.
Y-Chaining means that:
(1) when a cell is assumed to be a value
then all the other potential values of this cell must be removed
{2) when a cells is assumed to NOT be one of its potential values
and there are only two potential values for this cell
then this cell must be the other value.
The Forcing Chain consists of a chain of implications based on simple rules. The details of the Forcing Chain are given below.
Details of the Forcing Chain:
(1) If A6 contains the value 8 then A6 cannot contain the value 3 (the cell can contain only one value)
(2) If A6 does not contain the value 3 then A5 must contain the value 3 (only remaining position in the box)
(3) If A5 contains the value 3 then F5 cannot contain the value 3 (the value can occur only once in the row)
(4) If F5 does not contain the value 3 then F5 must contain the value 4 (only remaining potential value in the cell)
(5) If F5 contains the value 4 then D6 cannot contain the value 4 (the value can occur only once in the box)
(6) If D6 does not contain the value 4 then D6 must contain the value 8 (only remaining potential value in the cell)
(7) If D6 contains the value 8 then A6 cannot contain the value 8 (the value can occur only once in the row)