meaning - What does it mean when a piece of clothing cinches you in?

I've recently heard a girl saying:

"the shape (of a dress) cinches you in."

So the next day, I tried to look up the meaning of it but I've only found the word a cinch which is supposed to be a strap that holds a saddle on a horse.

The dress was tight around the waist area so I thought it can either mean that the dress is tight enough to hold everything (you know what I mean) in place or it makes you look slimmer.

I'm still not quite sure about what she meant so I'd appreciate your answers. :)