I prefer to use a different description.
Relax the lower back.
Here are two photos. The first one shows a common posture that I see in beginning students -- the butt sticks out in postures such as Single Whip.The second photo shows a more "centered" tailbone. The lower back is relaxed.
When most of us stand up, our lower backs are tense. They are concave, bowing inward toward the navel. But when you work on the body mechanics that are essential for quality internal arts (Taiji, Xingyi and Bagua) you learn to relax the lower back.
When I teach this, I have students put a hand firmly on my lower back. I stand normally. They can feel it bow inward. Then I relax and the lower back "fills up." At the same time, the hips drop down and inward. By relaxing the lower back, you are "tucking the hips," and no tension is required.
Relaxing the lower back is crucial for the opening and closing involved in movement. Bowing and unbowing the back is essential for silk-reeling and for storing and releasing in fajing.
And it's crucial for a centered stance in the internal arts.