You took the tile someone was adding to a pung.
Another player promotes an exposed pung to a kong, and that fourth tile is the one you needed. You take it and win.
It is one faan. The tile is claimed out of their meld rather than from the discard pile, which is why it has a name of its own.
Nothing here is planned. What you can do is notice exposed pungs around the table. If an opponent shows a pung of a tile you are waiting on, a fourth copy may still arrive, and your wait is quietly live in a way the discard pile does not show.
Trying to rob a concealed kong. Only a pung being promoted to a kong can be robbed. A kong declared from four tiles held in hand cannot be, with one exception: a Thirteen Orphans hand may rob even a concealed kong.
It adds its one faan to the hand you completed, and it implies the winning tile came from another player rather than from your draw, so it does not combine with Self Draw. It sits comfortably beside Concealed Hand if you had claimed nothing else.
Can I rob a concealed kong?
Normally no. The exception is Thirteen Orphans, which may rob a concealed kong because it is built from single tiles.
Does the kong still stand if I rob it?
No. The kong never completes. You take the tile and the hand ends.
Who pays?
The player whose kong you robbed, as though they had discarded the tile to you.
Play Mahjong Winds
Every win shows its faan line by line, using the table on this page.