Imagine you want to scare a 馬 horse that is walking buy. You get a 炮 firecracker and set light it and run away. The 馬 horse walks by and far past the 炮 firecracker that has a very large fuse. Eventually it goes off at a place that is so far 後 behind the 馬 horse that the 馬 horse does not even notice it. Your friend, which was standing by, smugly points out that the 炮 firecracker fuse is too long so the timing isn't right and you should have cut it shorter. Well, no shit. This is the very definition of 馬後炮 belated advice.