Did you know there are only two positions for the human body in which the pelvis is completely free and not being tugged on by a muscle? The two positions are vajrasana/virasana (sitting on your heals) and table position (being on all fours.) In any other position, sitting, standing, or laying down (whether on your back or front), the pelvis is being pulled by muscles off its neutral position thereby making it nearly impossible for a yoga practitioner, especially a beginner, to find what a neutral position is for him/her. Yet, finding a neutral pelvis is integral to finding your neutral spine. If the pelvis tips too far forward, the lower back gets too much of a curve. If it tips too far back, the lower back is flattened.
So, how do you teach students how to feel a neutral pelvis? Put them in virasana on a block. In Virasana, we sit between our feet. Put the block lengthwise underneath your sit bones. The block will make Virasana comfortable for most students. If a student still feels discomfort in the knee, add a block or place a blanket under the block. The benefit of using a block is the hardness of the block helps the student really feel her sit bones. If the blanket is over the block, that feeling will be lost.
Feel your sit bones on the block. Take a deep breath and close your eyes to bring more awareness inside. Slowly tip the pelvis forward, stop when you begin to feel the lower back harden. Bring the pelvis back to neutral and slowly tip the pelvis back until you feel the abdominals harden. You will not have to go far either way. Your neutral position is somewhere between these two points where both the abdominals and the back are soft and relaxed.
Then have your students lengthen from the sit bones through the top of their head. Have them do it slowly, bringing it up their body from their sit bones. Have them pay attention to maintaining the pelvis in its neutral position, without creating hardness in the abdominals or the back. (We have a tendency to create hardness as we lengthen by lengthening the front or back body faster than the other thereby tipping the pelvis.)
Now your students can experience the neutral pelvis, and its effect on the posture of the torso, and can bring that experience into other poses.