Eeep, this is actually VERY dangerous. You could crush her trachea, do major damage to her throat and vocal cords. You're also cutting off oxygen to the brain, which can cause gradual brain damage if you're doing this alot. It's very very very dangerous to cut off someone else's breathing, particularly if you're doing it during sex, when you're distracted and might not notice if she passes out, or shows other signs of distress. At the very least, if you're going to do this, make sure it's in a well lit area and you keep a phone near by to call for help if something happens.
Also, legally, this is tricky. It'd be a good idea to make sure that you get written, signed, and notarized (sp?) consent to this activity, just in case your grrrlfriend gets seriously injured/dies and you end up in court.