You probably already know this technique but you might not have known that it is
called a LOCKED CANDIDATE.
Locked candidates alway occur in the intersection between a box and a row (or column).
In this example, the 8's are confined ('locked') to the 3rd mini column in box 1, thus eliminating
the rest of the 8's in the column outside the box.
The other possible locked candidate situation is when a candidate is confined to a column
or row and the eliminations are done in the intersecting box (click here for an example)