Can someone explain what the central broadcast checkpoint is?

Can someone explain to me what the central broadcast checkpoint is? Is it a special node that SK runs? What powers over regular nodes does it have?

See this interview:
SK talked what is checkpoint system in this interview. Tyke’s book includes this interview’s transcript.

Vitalik Buterin [08|Aug 09:31 pm]:I saw somewhere, whether in the source code or the discussions, that you were working on some kind of improved checkpointing system for ppcoin and/or primecoin
Vitalik Buterin [08|Aug 09:31 pm]:something not quite so centralized
Sunny King [08|Aug 09:32 pm]:yeah its already in primecoin, it’s an updated version of ppcoin’s checkpoint system
Sunny King [08|Aug 09:33 pm]:even ppcoin’s checkpoint is not meant to stay centralized forever
Sunny King [08|Aug 09:33 pm]:and it’s going to work similarly in ppcoin in the future as well
Vitalik Buterin [08|Aug 09:34 pm]:so how does this new checkpointing system work?
Sunny King [08|Aug 09:36 pm]:basically developers can broadcast a checkpoint to the network, and if an user turns on the checkpoint enforcement in the node then it would follow the block chain fork of the checkpoint
Sunny King [08|Aug 09:37 pm]:this means that if the majority of network enforces the checkpoints then developer has power to thwart a sustained 51% attack
Sunny King [08|Aug 09:38 pm]:while the network is turning into a temporary centralization mode
Sunny King [08|Aug 09:38 pm]:although there are built-in checks to ensure even in the checkpoint mode developers cannot arbitrarily abuse his power
Vitalik Buterin [08|Aug 09:38 pm]:how did the system work before?
Sunny King [08|Aug 09:39 pm]:in ppcoin right now the checkpoints are enforced by default so users have no say in whether to follow it or not
Vitalik Buterin [08|Aug 09:40 pm]:ah, okay, so hardcoded into software essentially
Sunny King [08|Aug 09:40 pm]:yeah but it will be switched over in the future
Vitalik Buterin [08|Aug 09:41 pm]: although there are built-in checks to ensure even in the checkpoint mode developers cannot arbitrarily abuse his power <- what kinds of built-in checks are you talking about?
Sunny King [08|Aug 09:41 pm]:the system is designed because threat of 51% attack is real with altcoins
Sunny King [08|Aug 09:42 pm]:there is a consistency check with checkpoints,
Sunny King [08|Aug 09:43 pm]:meaning that developers cannot issue conflicting checkpoints and force double-spending on the network
Sunny King [08|Aug 09:44 pm]:for example, say developer checkpoint is issued on a block with 6-confirmations, then you can treat the transaction confirmed earlier than the checkpoint is safe
Sunny King [08|Aug 09:45 pm]:he cannot invalidate that checkpoint and ask the network to go into another block chain fork
Vitalik Buterin [08|Aug 09:45 pm]:what if a developer sends two conflicting checkpoints to two parts of the network at the same time?
Vitalik Buterin [08|Aug 09:46 pm]:so half picks up one first, half picks up the other
Sunny King [08|Aug 09:47 pm]:that would cause the network to fork and requires manual intervention like restarting or upgrading client
Vitalik Buterin [08|Aug 09:48 pm]:okay, makes sense
Vitalik Buterin [08|Aug 09:48 pm]:if it was perfect, you would just use it instead of the pow/pos in the first place
Sunny King [08|Aug 09:48 pm]:in primecoin user could also then ignore checkpoint and just follow the fork with more work
Sunny King [08|Aug 09:50 pm]:checkpoint is a temporary centralization defense against 51%, not on the same level as PoW or PoS
Vitalik Buterin [08|Aug 09:50 pm]:is it a jsonrpc command to do this, or are you planning on adding a GUI as well?
Sunny King [08|Aug 09:51 pm]:oh the Qt now has debug window which can do all the rpc commands
Vitalik Buterin [08|Aug 09:51 pm]:right, forgot about that
Vitalik Buterin [08|Aug 09:52 pm]:so what are the next steps / near-term goals in primecoin development at this point?
Sunny King [08|Aug 09:52 pm]:but if needed we could add it as an option setting
Sunny King [08|Aug 09:54 pm]:I think infrastructure and marketing would be the top priority for primecoin, more exchange support, mining pools and so on
Sunny King [08|Aug 09:55 pm]:the team is also expanded quite a bit so primecoin shouldn’t lag behind while I work on ppcoin v0.4
Vitalik Buterin [08|Aug 09:57 pm]:are you planning on integrating the latest btc features eventually? (eg. payment protocol)
Sunny King [08|Aug 09:58 pm]:sure both primecoin and ppcoin should keep reasonably up-to-date with bitcoin features
Sunny King [08|Aug 09:58 pm]:right now primecoin is ahead of ppcoin in this regard
Vitalik Buterin [08|Aug 10:00 pm]:what is ppcoin 0.4 going to have? The advanced checkpointing system?
Sunny King [08|Aug 10:02 pm]:It’s mainly a refresh to bitcoin v0.8 features, but there could also be some adjustment on certain protocols
Sunny King [08|Aug 10:04 pm]:the checkpoint would be adjusted a bit also but it won’t be as decentralized as in primecoin yet