When a digit can go only in the same four columns across four rows, no other cell in those columns can hold it.
Where to lookA pointer first: look at row 8. Where can 4 still go?Locked candidatesIn row 8, 4 can only go in column 1, column 2, or column 3, and those three cells lie in the bottom-left box. So no other cell in the bottom-left box can be 4.Locked candidatesIn row 9, 4 can only go in column 4 or column 6, and those two cells lie in the bottom box. So no other cell in the bottom box can be 4.JellyfishIn row 1, row 3, row 6, and row 9, the 5s all lie in column 1, column 3, column 4, and column 6. So 5 can go nowhere else in column 1, column 3, column 4, and column 6.
Pick a digit and find four rows where it has two to four places, all within the same four columns. The four columns are used up by those rows. Remove the digit from the other cells of the four columns. Swap rows and columns for the other direction.
This example, step by step
Where to lookA pointer first: look at row 8. Where can 4 still go?
Locked candidatesIn row 8, 4 can only go in column 1, column 2, or column 3, and those three cells lie in the bottom-left box. So no other cell in the bottom-left box can be 4.
Locked candidatesIn row 9, 4 can only go in column 4 or column 6, and those two cells lie in the bottom box. So no other cell in the bottom box can be 4.
JellyfishIn row 1, row 3, row 6, and row 9, the 5s all lie in column 1, column 3, column 4, and column 6. So 5 can go nowhere else in column 1, column 3, column 4, and column 6.
In hints
The hint first gives a direction: those four rows. Blue ground marks the rows, blue marks where the digit can go, and orange strikes remove it from the other cells of the four columns.