For the block to be excepted.
1. Hash the hash of the previous block.
2. Take the first six bytes of that. Let's call that rnd_selector
3. Take the current time since the last block in millis i.e. 100000
4. Take the effective balance of the account that generated the block.
If balance * time_in_millis > rnd_selector then the block is valid.
My understanding based on
this code is that I can sign a new block if:
base_target * my_balance * time_in_millis > SHA256(previous_block_signature + my_public_key)
So you can very easily iterate through thousands of possible current block signature such that it is guaranteed that you will be the person able to mine the following block. Since the whole process is transparent (public keys and balances are known to everyone), a moderate amount of computation power and coins assures that once you get to create one block you can sign all the blocks following it.
I'm not an NXT developer so I may be wrong.
What do you mean by iterating through thousands of possible current block signatures?