Quoted from Mr_Tantrum:Dirty pool is a fun/rare achievement that I've only done once. When starting a multiball it is possible with just the right timing (it's more by accident than intentional) to trap a ball behind the target gate just before it rises after defeating a mothership. Now you have a ball behind the targets where you can then bash the targets with a regular ball and transfer the momentum to the ball behind the targets and into the standup target/opening.
I believe that AFM is not alone when it comes to Dirty Pool, but I've personally not experienced it on any other game.
Here is an example on a virtual pinball game:
The term "Dirty Pool" in pinball machines originated from the 1993 Bally's pinball machine, "The Addams Family." In the game, there's a shot called the "Vault" that's usually blocked by the "Bookcase." However, if a player manages to get the ball into the Vault without opening the Bookcase (by spelling the word "GREED"), Gomez (one of the game's characters) says, "Dirty pool, old man. I like it!"
This phrase became popular among pinball enthusiasts and is now used to describe a shot that's technically not supposed to be possible but can be achieved through skill or luck. So, when someone pulls off a "Dirty Pool" in pinball, they've essentially done something that's considered "impossible" within the rules of the game, making it an impressive feat!
(Thanks Pi.ai!).
btw IJ has this as well if you lock a ball with the drop target up (airball) it says "You cheat Dr. Jones!".