The rare enemy effect is a niche but interesting phenomenon in predator-prey interactions.
Consider the case of earthworms. Their number one enemy is the mole (a mole eats almost its body weight in worms each day). So the worms have adapted to recognize an approaching mole by sensing the vibrations of its feet as they dig in the soil. When they sense it, the worms escape to the surface, where mole encounters are unlikely.
However, other predators of worms have figured this out. For example, seagulls perform a "dance" to vibrate the ground, bringing up unwitting worms to the surface, ready for snacking. Wood turtles do a similar thing. Humans, in a process called "worm charming", plant a stick into the ground and vibrate the stick to summon the worms, often to collect them for fishing (but some do the process for fun and sport).
The rare enemy effect happens when Predator A exploits the prey's anti-predator response to Predator B (the main predator). It must be that Predator A is a rather rare encounter to the prey (hence the name of the effect); if they were a main predator, the prey would eventually develop an anti-predator response to them, too.
If you look closely, you can see a similar effect in the human realm, too:
And do not think that these processes are limited to capturing your money; they want your support, information, attention and time, too. If there's a predictable anti-predator response, there's someone who exploits that response, legally or illegally.
There are many more concepts in my "Mind Expander" tool (it's free)