When a digit can go only in the same three columns across three rows, no other cell in those columns can hold it.
Where to lookA pointer first: look at the right box. Where can 5 still go?Locked candidatesIn the right box, 5 can only go in row 6, column 7 or row 6, column 9, and those two cells lie in row 6. So no other cell in row 6 can be 5.SwordfishIn row 1, row 5, and row 8, the 9s all lie in column 1, column 3, and column 4. So 9 can go nowhere else in column 1, column 3, and column 4.
Pick a digit and find three rows where it has two or three places, all within the same three columns. The three columns are used up by those rows. Remove the digit from the other cells of the three columns. Swap rows and columns for the other direction.
This example, step by step
Where to lookA pointer first: look at the right box. Where can 5 still go?
Locked candidatesIn the right box, 5 can only go in row 6, column 7 or row 6, column 9, and those two cells lie in row 6. So no other cell in row 6 can be 5.
SwordfishIn row 1, row 5, and row 8, the 9s all lie in column 1, column 3, and column 4. So 9 can go nowhere else in column 1, column 3, and column 4.
In hints
The hint first gives a direction: those three rows. Blue ground marks the rows, blue marks where the digit can go, and orange strikes remove it from the other cells of the three columns.