Find two cells in the same row, column or box whose candidate lists are identical and exactly two digits long — say 4 and 7. One of them is the 4 and the other is the 7. You do not know which, and you do not need to.
What matters is that between them they have used up both digits. Every other cell in that unit can drop 4 and 7 from its candidates, which frequently leaves one of them with a single candidate.
The technique never fills a cell directly. It is an elimination — its value is entirely in what it lets you remove, and that is what makes it feel unproductive right up until the cell it unblocks.