At the start of every round each pair must inform their opponents of their basic system (e.g. strength of no trump opening, 4 or 5 card major, forcing club) and their carding methods and in particular any unusual openings for which opponents may need to prepare.
Announcements in the following 3 instances should be made each time the bid is made:
The purpose of alerting is to advise your opponents that your partner's call has a special meaning.
The alert is made by the partner of the person making the alertable call. It is the responsibility of the alerter to ensure the opponents are made aware that an alert has been made.
Alert any call of your partner which you believe to be alertable even if you can't explain its meaning.
During the bidding no call above the level of 3NT is to be alerted unless it occurs on the first round of bidding. Calls above the level of 3NT after the first round of the auction are to be alerted at the end of the bidding. The dummy or declarer alerts the defenders before the opening lead. The defenders alert after the opening lead has been made but before it is faced.
The correct way to ask for information following an alert is to say "Please explain" You may only enquire when it is your turn to call.