SpreadX11 is different from plain X11 by introducing a sophisticated pool prevention mechanism.

With SpreadX11 every block header contains additional information (MinerSignature and hashWholeBlock). With the help of this information the protocol ensures that the miner of a new block is always also the first one to know the content of the whole block and the private key to spend the coinbase transaction. (contrary to pool mining where the pool operator is the first one to know those things)

So when a miner finds a block, he must himself sign and transmit the block to the network (like solo mining), instead of having a pool handle this for him.

This effectively prevents pools by making their rules non-enforceable, since any miner in any assumed pool can always just steal the block reward instead of following the rules set up by the pool.