If the pistol is empty, that is no round in the chamber, then you may drop and leave the empty magazine. You must retain the magazine only if it still has rounds in it OR if there is still a round in the chamber.
You might want to review the rule book, which you can download from www.idpa.com.
There is a very good discription therein of the various reloads which are legal in IDPA competition, ie. slide-lock reload, reload-with-retention and tactical reload.
--Lin