Flush (out): To cause (as a bird) to flush; to expose or chase from a place of concealment <flushed the boys from their hiding place>
Flesh (out): To make fuller or more nearly complete —used with out <museums fleshing out their collections with borrowed works>
Many people tell me they are going to “flush out” their ideas. I suppose this isn’t wrong, because it means they’re going to bat the ideas out of the bushes where they are hiding, like pheasants. But what they really mean is “flesh out,” i.e., putting flesh on a skeleton.
Flushing is like what you do with a toilet—getting something to leave where it currently is (though I guess we don’t want toilet contents to come out into the open). Fleshing is like what you do when you’re gaining weight—adding to what’s already there.
I won’t bother defining “flash”: seems like most people have the hang of that one.