A CDJ 900 is a CD player

The DJM 900 is a mixer
I'm going to assume thats what you were asking about.

I haven't had the pleasure of using a DJM-900, but I've used plenty of other mixers that had a similar function.

The mic section is controlled by a THREE way switch
-- OFF
-- ON
-- Talk Over

On some mixers, the "Talk Over" is called "Duck"

When you are at the club, and you use the "Duck" setting, the mixer will automatically LOWER the levels of the rest of the program (i.e. the music). This saves you the trouble of doing TWO things
-- Turning on the MIC
and
-- Lowering the level of the music

The reason for lowering the music is to allow your voice to be "louder" than the music so that your voice an be "clearly heard" over the music.

It really depends on what you are used to doing work flow wise.

I am used to BroadCast radio, and thus am in the habit of "lowering" the program levels while raising the mic levels.

But that DOES require more "action" on the part of "things to remember to do.

If you have a mix going with MULTIPLE tracks playing on top of each other (i.e. more than one channel slider is UP and has music), lowering the "music" and turning on the mic may require three or more hands!
Thus the "duck" feature saves the DJ a ton of work (to say nothing about the alien surgery required to graft on extra hands)

Ah I see. And yeah oops I meant DJM lol. Thanks!

i wouldnt use talkover..its to abrupt..dont sound good..i leave my mic open for the 2 hours i broadcast. though that depends on how much noise you have around you and what mic you are going to use,,condenser mics are the worst for picking up everything,,cardiod are better if you want to leave the mic open.if you are going to be broadcasting you can tape across the fader chanel halfway down.then you dont even need to look down at what level you have pulled the fader down..thats if your not confident on judging the levels..just set the first track with that method then your voice will be the same level approx..but you cant use auto gain because it will throw of the level fader you have set..loads of ways to do it really

Soul83 is right - the "duck" is pretty abrupt

It all depends on what your reason for getting on the MIC actually is...

Let's just say the DJ is on the mic to hype up the crowd or something, what would the dj do in that situation?

depends what kind of club it is..if it the wheel tappers and shunters type club like some of our northen brethen on here play in.they normally have a dj console with the mic attached..in london you hold the mic and might have an on and off switch..though some dont know what an off switch is due to the fact they cant shut up for more than 3 seconds..sometimes less is more...using a mic in any situation just depends on your style..i can really say what you should do personally

One of the issues is that is really hard to tell (at home) how dramatic / abrupt the level drop is when the mic mixer goes in to "duck".

In general, the LOUDER it is (the main PA), the more noticeable that the duck is going to be. Depending on the club - it may sound like half the amps just shut down.

If the DJ is trying to make an important announcement (i.e. mobile / wedding - telling table 7 to enter the buffet line) then using the DUCK can garner "attention".

In a packed club with a jamming floor - the duck might have bottles flying your way....

If the goal is to cut in over the music with some "drop in", I'd say just use the ON and fiddle with the channel gain when on the mic.

this is why holding the mic is not for everyone,thats why you have an mic man and a dj..you either have that structure in your voice and the flow or you dont..you can get way with it more on radio.auto ducking has an equally bad rep,but using the mic output on a controller is a no go,their pretty rubbish from my experience..lot of factors involved its not just buy a mic and your good to unless you have a great voice then you can get away with most mics,but you may have to go through a few mics till you find one that suits your voice..then you have to find a mixers output that compliments all that..

One could use microphone processor to change how the mic sounds...

dbxpro.com

more gear to the gig !