This could take many forms. The most common in the real world is imagine an older person preparing to make their will. They have two children named Sam and Sue who they love equally. They want their assets to go to Sam and Sue equally as well. As the parent ages, Sam begins to take care of them while Sue does not because Sue lives in another state. Sam uses this opportunity to convince the parent that Sue clearly does not love the parent as much as Sam (even though it isn’t true) and should be cut out of the will.
It is unfortunate, but as people age, they become easier to control. This can open the door to duress where the child in the example would force the parent to give all assets to them and cut the other child out.