This seems to be a love/hate thing in the recording community, but top down mixing. It's basically replicating a mastering chain on your master bus by adding a compressor/limiter/EQ, any kind of plug-ins for mastering like Ozone and that whatnot.
People object to it because when you send the track off to be mastered and remove all those things, your mix is going to send entirely different then when you were tracking it and hearing it with everything on that master bus and who knows if the guy who ends up mastering it in the end is going to pull off what you got out of it, you're own way.
I agree with that if you're doing some heavy processing on the master bus and if that's the case, you need to dial in your instruments better so you don't have to do that.
I use a Brainworx Townhouse Buss Compressor (SSL) and if I need something a little louder because I was overly cautious about clipping, Brainworx masterdesk or limiter to do it. I don't even tweak the Townhouse, there's a preset called Mix Startup or something and it does everything I want/need it to. I'm just looking to glue the instruments together lightly. The rest of the glue I really try to get with the tones of the instruments themselves, where one is ending the other begins kind of thing.
It's not invasive enough that if I take it off, the tracks sound different, but it gives me a great idea of what things are going to sound like once mastered if it's a clean master. I was using SSLComp for a long time but this Brainworx doesn't color it nearly as much as the SSLComp.....even though they're both SSL-based bus compressors.