Originally Posted by Gia al Qamar

It's an unfortunate choice of words that some teacher's use to try to get students to lengthen their spines but pointing the tailbone to the ground.
You are not actually 'tucking' the pelvis under, but rather are using the psoas muscles (they're located in the lower abdomen and run around the outside of your groin) to push the tailbone so it is pointing to the ground...the buttocks aren't involved at all.
Think of pulling your belly button in to your spine, but not over-using your abs and completely disengaging your tush!
Gia