As someone who rides horses,..
and is poly,...I must say, I really hate this terminology. lol
As for the actual 'threat' of such things. I am very much a fence
-sitter on this one. I have been in a situation where my then-secondary eventually tried to extract me from my relationship with my primary.
There was no doubt about it. This is not some debatable he-said, she-said type of thing. It is something that became very obvious to all involved as time went on.
However, there was ZERO doubt in my mind, about the outcome, of trying such nonsense. I cared about my secondary very, very much, but no one, can convince me to walk away from my Primary.
This all boils down to communication. As everything does in poly.
The only way I can see someone being able to 'extract' a person, is if said person is not being totally forthright about things said and done, between all involved.
It may be as simple a thought process, as thinking that they wont tell others in their life, certain aspects (of things said and done by the 'puller')because they don`t want to cause a 'rift' or argument between their loved ones.
As soon as they start to 'protect' the person causing doubt, then things can snowball.
In the end, we all reap what we sow. If someone or a group gets 'left', then the person doing the leaving wasn`t willing to be objective for whatever reasons.
I don`t believe regular people with common sense, will leave if it truly feels right. Some people are easily manipulated,...but someone who is easily manipulated will always cause those that care about them, insecurities anyhow,....
People leave relationships for others, because they believe they have found something better, period.
Inside, or outside of poly, thats the cold truth.