Institut d'émission is not a central bank (banque centrale.) Literally, it means "issuing authority". While a central bank manages a currency - including cash - an institut d'émission issues and withdraws banknotes and coins of a currency that does not need to be managed, because it is linked to another currency. The institut d'émission would hold, e.g. french francs and issue 100 CFA francs for a french franc on demand. They would buy 100 CFA francs for a french franc on demand. If reserves of french francs would go down, e.g. because the country runs a trade deficit it would diminish the amount of CFA francs outstanding. If the country would receive more french francs, e.g. through foreign aid, the institut d'émission would have the possibility to increase the supply of CFA francs.
Peter