It doesn't jump start labor. It helps to progress you from an inactive labor to active. Orgasm will work.
The sperm is what helps to ripen the cervix, but masterbating will help with contractions when you orgasm.. there are other ways to help jump start labor if you wanna get to that point.
I heard it's the sperm that helps with starting labor.
Yea sex is great way to start labor that's why in your 3 trimester dr will tell you to be more carful but I do t know if masterbation will work the same way