My teacher John Kells always summed it up as "bum in", which to be honest was not a particularly helpful instruction. Many people resort to a forceful tucking under of the pelvis in a desperate and tense attempt to have a straight back.
If you are not careful this can lead to other problems in joints such as the knees. Don't believe me, investigate it. I like the "ideokinetic" approach in the article.
Tucking the Sacrum