A single dolphin kick is allowed during the underwater breaststroke pull-down, but it must be taken AFTER the pull is initiated. USA Swimming has issued an updated interpretation of the rules; this interpretation will also apply in USMS competition. It deals with how judges can determine when the pull has begun. Please read the document in full, but basically the big difference is that a downward motion without hand separation is NOT taken to indicate that the arm pull has been initiated.
There are two basic approaches — early and late — to the timing of the dolphin kick during the breaststroke pull-down; GoSwim has a great video, posted below, illustrating what is legal and what is not. Especially if you plan to do an early kick, BEFORE the bulk of the pull, you should watch the video carefully to make sure you are not disqualified.