recently my gf has been teasing me about these other "spots"

i know from my own experience/knowledge of the

clit
gspot
ears
neck
hands
feet
legs
thighs
butt
stomach
nipples
breasts
back

But, now I hear that there's one on the small of the back..more towards the butt. Where is it? How can I find it? and how would I properly stimulate it?