Within a row, column or jigsaw piece, each number can only appear once. This means that if there are two numbers that only appear in the same two cells within that area, then these cells can only contain one of these two numbers.
As a result of this, any other pencil numbers can be removed from these cells.
For example, the highlighted cells are the only two places a 3 and 6 can be placed within their jigsaw piece:
As a result, these cells must contain one of these numbers and all other pencil numbers can be removed: