You won on the tile drawn to replace a kong.
You declare a kong, draw the replacement tile from the back of the wall, and that tile completes your hand. It is one faan, and the name in Chinese means a flower blooming on top of a kong. The replacement is a genuine draw, so this also counts as a self-draw.
Any kong gives you an extra tile from the wall, which is a free look you would not otherwise get. When a kong does not damage your shape, declaring it is usually correct. When you are one tile from winning, that extra draw is a real chance rather than a formality.
Declaring a kong that wrecks your wait. Turning a pung into a kong removes a tile that may have been part of a better shape, and it exposes information. The extra draw is worth one faan only if it does not cost you the hand.
It scores alongside Self Draw, because the replacement is drawn rather than claimed, so those two alone are two faan. It also stacks with whatever your hand was already worth, and with All Kongs, where four kongs is a limit hand in its own right.
Does it count if the replacement completes a second kong?
You draw again. The row applies to the tile that finally completes your hand.
Does it work after a concealed kong?
Yes. Concealed and exposed kongs both give a replacement draw and both can score this.
Does it stack with Self Draw?
Yes. The replacement is a draw, so both rows count, for two faan.
Play Mahjong Winds
Every win shows its faan line by line, using the table on this page.