How will Winnie help the Tucks in Tuck Everlasting?

Winnie helps Mae escape from jail after accidentally killing the man in the yellow suit.

The man in the yellow suit is obsessed with finding the spring that turned the Tuck family immortal.  He tracks them down from a rumored family story, and the sound of Mae’s music box.  He then hears them tell Winnie their sad tale.  Unfortunately, they have no idea he was listening.

[Through] it all, not one of them noticed that the man they had passed on the road, the man in the yellow suit, had crept up to the bushes by the stream and heard it all, the whole fantastic story. Nor did they notice that he was following now … (Ch. 8)

The man follows them to the Tuck house.  His intention is to find the spring and get rich off of it, selling people the dream of being immortal.  The Tucks tell no one about the spring because they do not consider being stopped in time a good thing.  All the man in the yellow suit thinks of is greed.

Mae is so surprised to see him and learn that their secret is out that she just reacts.  He had told her that he was there to get Winnie, but that was just an excuse.  He managed to go on ahead of the constable, who was going to slowly for his taste.  She gets upset when he says he is going to take Winnie.

With a dull cracking sound, the stock of the shotgun smashed into the back of his skull. He dropped like a tree, his face surprised, his eyes wide open. And at that very moment, riding through the pine trees just in time to see it all, came the Treegap constable. (Ch. 19)

Mae Tuck did not mean to kill the man in the yellow suit.  She is arrested anyway.  The problem is that if she is hanged, she won’t die.  The Tuck's secret will then be out.  Winnie knows that, and she wants to help Mae Tuck break out of jail.  Miles has a plan, but Winne's plan involves Winnie changing places with Mae so that the constable won’t find out she’s gone until she is long gone.

"I can help! When your mother climbs out the window, I'll climb in and take her place. I can wrap myself up in her blanket, and when the constable looks in, he won't be able to tell the difference. Not in the dark. I can hump up and look a lot bigger….” (Ch. 22)

Jesse asks Winnie to wait until she is his age and then drink the spring water to become immortal like them.  He wants to spend his life with her.  The Tucks vanish with Mae, and years later we learn that Winnie did not drink from the spring, and died a natural death.