Interesting.
wait...who blows up the balloon?
lmao! It's done with a hand pump.
I actually have looked into this, before pregnancy in fact when I was studying health at college.
I decided against it. It stretches the opening of the vagina which includes the perineum and yes, this would help stop tearing. However it's the after effects that worry me. If you look at people who stretch their ears for example, they have to slowly stretch their ears over time to get those big gaping holes. When they decide to take the rings out of their ears, it can take a very very long time to go back or sometimes it doesn't at all. This would be the same in my opinion. If you stretch the vaginal opening over time then even before you give birth it's going to be quite loose and it would stay very stretched afterwards too. You'd have to put a hell of a lot of hard work and effort to get back into shape iykwim!
If you don't stretch down there, the skin will be torte and when the baby is crowning, you may tear or you may not. If you do tear or get cut then you get stitched up, your downstairs will swell due to the injury and that will cause you to heal up tight. If you don't tear then when your perineum stretches it will cause teeny tiny little cuts where the tissue is expanding rapidly. After birth, the body will see this as injury and then swell- causing you to heal up tight.
If you're already that big then it's no big deal for the skin and therefore wouldn't need to heal.
Also, if you've ever overstretched a muscle, when it heals it feels tight. If you've done dancing or gymnastics- you have to stretch regularly to keep supple.
Ok i've written an essay but that's my best way to describe my theory lol! Long term stretch stays stretched, short term stretch goes back to normal.