What Treyarch didn't want you to know...the zombies Ranking system. It's been a mystery for a while now and nobody really knows how it works. Truth behold, It's rather easy. In this post you will:
A.Figure out how to rank up

B.Figure out why you get De-ranked

Now go to the next section and start learning!

Think of the Emblems and Slashes like Multiplayer. The Emblems are prestiges and the slashes are levels. You get 5 Slashes per prestige. After the fifth slash, you basically prestige. The prestiges are:
Prestige 1-One bone
Prestige 2-Two bones
Prestige 3-Skull
Prestige 4-Skull with blue eyes
Prestige 5-Skull with dagger
Prestige 6-Skull with blue eyes and dagger
Prestige 7-Skull with shotgun
Prestige 8-Skull with blue eyes and shotgun

NOTE:People have claimed to have a skull with shotgun and skull with blue eyes and shotgun. If you have this message me on xbox :"Tig Lobbys" so I can confirm the shotgun emblem. CONFIRMED

The ranking up system has to do with the deranking system somewhat. Anybody ever heard of league play? That's right, black ops zombies ranking system is like the ranking system of league play! You probably thinking to yourself "what and idiot this guy is, thinking league mode has something to do with zombies. Well actually, if you've every played league mode and has been demoted from lets say...Gold to Silver, that's how zombies work. If you ever had been demoted, IT'S FROM NOT PLAYING. Now that that's cleared up, ranking up. RANKING UP HAS TO DO WITH SKILL LEVEL. If you have a constant 20 round-ratio, you'll gain ranks faster than a constant 10-round-ratio. Now don't expect to rank up so fast. It may take a while to rank up, so don't get your head down. They also RANK YOU BASED ON YOUR OVERALL STATS OF EVERYTHING YOU DO IN THE GAME. So for example, if you open a shit ton of doors and kill a sh** ton of zombies and have a sh** ton of everything, you'll have a sh** ton of ranks....if that makes sense.

Now the deranking system is a little weird. It is true that is based off the players skill level (as confirmed by the creator of BOII on his twitter) BUT AS A TEAM. So if your partner gets downed 50,000 times then you're most likely going to go down a rank. Hope this helped and please reply and tell me if this helped you

Keep this topic alive and reply that way other people can see this post down the road and be happy too. Have a nice day and go kill some zombies.