Within a row, column or jigsaw piece, each number can only appear once. This means that if there are three numbers that only appear in the same three squares within that area, then these cells can only contain one of these three numbers.

As a result of this, any other pencil numbers can be removed from these cells.

For example, the highlighted cells are the only places 3, 6 and 7 can appear in this jigsaw piece.

As a result, these three cells can only contain these numbers, so all other numbers can be eliminated from them.