If a digit can go in at most three cells in each of three rows, all in the same three columns, the rest of those columns lose it.
On a Thermo board this works exactly as in Classic. The examples are from Thermo puzzles on this site.
Worked examples
A position from a real Thermo puzzle on this site. Step through it: the board lights what each step talks about, and the arrows show what rules what out.
Three rows, three columns
Look at three rows. Where does digit 5 appear?
Where it fits
ExtremeSwordfish is an Extreme technique: Extreme Thermo puzzles are the first to need it.
The Classic Swordfish lesson teaches it on a plain 9×9 board. The Thermo overview explains the rule and the levels.
Swordfish in the other styles: Sudoku X, Jigsaw, Windoku, Arrow.
Try it on an Extreme Thermo puzzle. Next Step points it out when it is the move.