On one episode on "The Real," Tamar explained to the ladies about this idea that you have your partner who doesn't seem to be paying attention to you or not all the way meeting your needs, then you have this person on the side that you don't like but they like you and you pretty much string along for the time being. It's almost like you have people at your service when you need them because they are side pieces. You may not necessarily sleep with them but you use them for what ever you need.
It's interesting that this is considered part of a girl code because females always get made at guys for having side pieces or multiple women while they are dating but I guess are saying we should do the same thing but call it something different. So many women use the plus one rule and probably don't realize they are in that situation. She could get into a fight with her partner and then she starts flirting with someone or being vulnerable to someone that could either be in the friendzone or she can't stand but desperately needs someone to vent to. Then she could lead them on and go back to her partner because she got what she wanted. Sounds shady, but it can happen subconsciously.
The clip above is Tamar talking about the plus one rule and the clip below is her talking about her "engagement" during her plus one experience. Enjoy!