Article here:

https://blogs.psychcentral.com/psychology-self/2019/10/triangulation-and-narcissism/

Basically, for example, it's when a guy tries to give you a sob story about his wife (she's jealous, controlling, gotten fat, etc) using that to make you more susceptible to his covert advances.

Or your bf/love interest telling you about another girl or female friend (real guys that like you won't do this) but talking about how pretty she is, how she's a great listener or has a good personality, etc. Basically in both cases the manipulator is making comparisons between both in order to make you jealous or to compete for his attention.

You have to be aware of this, it can be pretty effective.