Cervical Position Knuckle Rule

Lainey • Due with Baby #1 Nov. 2016!
Okay so this is probably going to sound ridiculously stupid but I figure if I can't ask here, where would I ask?
Has anyone heard of this knuckle rule? Apparently your cervix is high if your finger goes in to your third knuckle, it's medium if your finger goes into your middle knuckle, and it's low if your finger only goes into the first knuckle.
Right now I'm fairly sure my cervix is high, closed, and firm. Ok, fine.
Here's where my question gets stupid. When your cervix is low (during AF), supposedly you can only insert your finger to the first knuckle. But I'm finding this confusing! If the cervix is that low, doesn't that mean you can't go past your cervix? How on earth can I put a tampon in, or have sex with only an inch of space?
I can't believe I don't know the answer to this at my age and am embarrassed to even ask, but if dog wove could explain this to me I would be really grateful!