I didn't have this, but as a vbac it was the only induction option offered in my last pregnancy so I did a lot of research.
There's some nice info here
Basically, this method "works" in two ways. On the one hand it can be a lot like a membrane sweep... the foley is inserted through the cervix and inflated/filled to lift the membranes off the cervix and stimulate hormone production. On the other hand, the foley is also pressing down evenly/firmly on the cervix in the hopes of manually "opening" the cervix itself. So the goal is a combo of dilation and hormone production, leading to the body kicking into birth mode and taking over the process.
It works for some mamas and not others but it's generally considered the safest induction technique for a vbac mama (since it doesn't involve outside medication). I've been told that it can be really uncomfortable, but a few women have told me they really didn't notice much discomfort.