Locked candidates
If every possible position for a digit in a box lies in the same row, that digit must be in that row within the box. You can eliminate it from the rest of the row outside the box. The same reasoning works for columns and in reverse.
Worked example
In a box, 6 can only go in two cells on the same row. One of those cells must contain 6. You can therefore remove 6 from other cells in that row outside the box.