Because responder's transfer bid sounds like responder has a suit other than the suit responder really has, it is necessary to alert the use of a transfer to your opponents.
This is done immediately after responder makes the transfer bid and before the next play in rotation bids. The notrump bidder simply announces, "alert" and plays the Alert card next to partner's bid. Only explain what responder's bid means if your opponent asks.
You may ask your opponents about the meaning of an alert or call at your turn to call as well as before you play to any trick.
