What makes a good plot twist?

[Jon UKX]

Yes, it's time for another one of Jon's What makes a topic in the writer lounge.
My question is What makes a good plot twist? Now, I have never done a plot twist before but I have a rough idea on an upcoming plot twist in my fic.
The plot twist is that Roxy (she was in chapter 1) returns and she meets up with the gang. Unfortunately, Jon falls in love with her and decides to leave the girls for Roxy and the rest involves a shocking surprise.
So what a good plot twist?

[Chris 2.1]

Well Jon, I believe your example is good, especially if Leah and Annie DON'T LIKE Roxy, that makes for a difficult choice for Jon. Weather HE leaves, or weather the story focuses on him is really down to you, but remember what the story's about!!

I found Granbull's death in FRTC a good twist, PERSONALLY. I also found that shootings can be fun: Al threatening to shoot Sandra really switched their characters around, developing Al into a lot more than a coward, and revealling Sandra as somewhat Timid, no?

I wonder what topic you'll do next?

ps: Attachment - FRTC spoiler. I think...???