Hi Ana,
Thanks for being here! When I say "pull your bottom ribs down to your hip bones" it's an image to help you keep your abs engaged when in an ab curl or a c-curve. In the ab curl, for example, sometimes our neck wants to overcompensate for our abdominals. You can tell this is happening if you're not lifted to the bottom tips of the shoulder blades. So by saying "ribs to hips", it's a way to keep you lifted and take the tension out of the neck. Additionally, it's a way to find more length in the body. By audibly connecting the lower body to the upper body, we can often times find more reach. Notice I don't say "hips to ribs" (the other way around) when in an ab curl because that would suggest pelvic tilting, which we don't want (in most ab curl exercises unless cued to do so).
You'll know if you're using your joints instead of muscles if your joints are locked. This is why it's always a good idea to unlock knees and elbows when pushing and pulling with legs and arms.
I hope that helps! :)
Cheers,
Gabrielle