Jellyfish

A Jellyfish occurs when in four rows all possible cells for a given digit lie in four columns. This digit can be eliminated from the remaining cells of these columns. The technique also works when we replace rows with columns.

In the following sudoku diagram in the second, fourth, sixth and seventh rows, the digit 3 can only be placed in the third, fourth, fifth and ninth columns (candidates highlighted in blue). You can eliminate the digit 3 (marked in red) from the remaining cells of these columns. After applying this technique, only the digit 1 can be placed in the R5C4 cell.