Context
The informant is a Chinese American. We were discussing interesting superstitions in Chinese or American cultures when she brought out this item.

Content
You’re not supposed to step on cracks in the floor. If you step on it, you’re going to break your mother’s back. And I think kids kinda play for fun with it when they’re little. There are very few kids who actually believe it. Obviously, because kids step on it all the time, and no one’s mother dies of that. It’s mostly just for fun.
Interviewer: And how do they play with it? Like in what situations?
Informant: Someone would say, don’t step on the cracks or you would break your mother’s back. And all the kids have to avoid stepping on cracks. They just have to all walk around like to avoid cracks,
Interviewer: And they just do it for fun?
Informant: Yeah, they just do it for fun.
Interviewer: Like, they laugh and walk around it?
Informant: Kind of. It’s more like if someone does accidentally step on a crack, they would point it out and like, ‘haha, you stepped on a crack; you broke your mother’s back!’ kind of thing. It’s obviously rude and stupid.

Analysis
First, the saying itself includes a rhyming between “back” and “crack”. This is probably how the crack in the road is connected with the mother’s back.
Second, the saying involves homeopathic magic. Stepping on a crack is likened to actually stepping on mother’s back.
Third, the kids make fun of the saying, because they don’t believe in it. There is a counter-hegemonic feeling involved. The kids are supposed to follow the saying even they don’t believe it, so they follow the saying in an exaggerated way: for example, they intentionally avoid all the crack, and make fun of the kid who accidentally steps on a crack instead of feeling worried for the kid’s mom.
My informant doesn’t believe in the saying. She thinks the saying is stupid. She also cannot understand the doings of the kids.