Deep six is a BO that, as I understand, was popular quite a while ago. Unfortunately, it basically requires you to be constantly out-pacing and getting in ridiculous shots with your units. I'm not sure on the exact build order, but I've learned it as a 1 fact FE into 2-rax, so you can defend the exp against an early goon push, make sure to get eng bay before that exp comes up, and keep on guard for a reaver/dt harass. Then, while getting upgrades for marines, you put down a second factory, and 4 more raxes, so you have 6 rax 2 factory. I then pump straight tanks and MnM from the two base, and try to push out when i have 1/1 bio and 1 attack on the tanks, which usually happens around 6-10 tanks and 2-3 groups of mnm.
Unfortunately, if your opponent spots this, you really need to switch it up and include fbats or some anti zealot, because that will counter your push and shut down the strategy. I basically go all-in for a contain at this point, and hopefully the opponent will be mostly goons up until now, and you will be able to push them back into their natural, slowly pushing tanks forward and setting up a static defense with bunkers/depos and turrets. If you can establish this contain, and avoid the harass, your next priority should be keeping ahead on upgrades, getting a third, and getting ahead by drops or mass tanks or something...
As a viable strategy, however, you're working uphill, the Protoss can simply spot that first push coming, and do a slow retreat back to his base while producing a ton of zealots. All he really has to do is be intelligent in that retreat, outranging the MnM, and picking off tanks if they get too aggressive, then bring in the half dozen zealots to break any chance of contain, and quickly teching to HTs, which basically spell GG. You would have to get in perfect stims, bring in 4+ scvs to establish a block against the zealots, while microing tanks to hit goons, and marines to pick off zealots while avoiding the hits that would basically end the push.
If you're feeling ballsy and you spot a quick expansion or really feel that he might go for a safer double expo after a reaver harass (which the MnM with tanks are very strong against if you can split them quickly), this could be a very strong push... Also, I've heard this is a very good late-game counter to mass carriers, but thats not really a bo, more of a ballsy move to get ghosts and MnM to try and lockdown carriers and destroy interceptors...
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Edit: I found a very good write-up of the strategy:
http://www.battlereports.com/viewreports.php?reportnum=1633It has a few good points I haven't ever really incorporated, and alot about how ghosts can really be an asset.