If you have >51% of the mining power, you can overpower the rest of the network. Simply pick a block in the past and start mining from there, ignoring everyone else. With >51% of the hashrate you _will_ overcome the rest of the network and enforce your new chain.
They can do it in secret. Build a secret parallel chain while they transact, then release the chain after they've got their BTC.
And 51% is not "insane". It's absolutely tiny. As I said, _any_ of the large pools could do it, right now.
If you have >51% of the mining power, you can overpower the rest of the network. Simply pick a block in the past and start mining from there, ignoring everyone else. With >51% of the hashrate you _will_ overcome the rest of the network and enforce your new chain.
They can do it in secret. Build a secret parallel chain while they transact, then release the chain after they've got their BTC.
And 51% is not "insane". It's absolutely tiny. As I said, _any_ of the large pools could do it, right now.