What 'exactly' is selfish mining?
If it just means that you build new blocks on top of the most recent block that you mined yourself, then that won't work for a long time unless you have at least 51%
So I guess it's something slightly more subtle; e.g. they'll try to build on their own block, but will give up every time they fall X blocks behind.
Is that it? And what is X?
I can see how that could work if you have less than 51% of the hashrate. Where 'work' is 'steal some block rewards and encourage other miners to switch off'