A common saying in tai chi is to "tilt the pelvis" or alternatively to say "tuck the tailbone". What does this mean? Simple. It's a way of straightening the lower spine by pivoting the pelvis and pulling in the tailbone. Unfortunately, this is often misunderstood to mean that we should use muscular force to put our body in a particular posture. Have you ever seen someone trying to do this? They push their pelvis forward, lean back, and end up in such an awkward position that they can hardly stand.
How can we teach this to beginners so they don't get into the outlandish postures we often see? It goes back to "sinking the qi". When we deliver energy, like with the final part of a Brush Knee movement, we shift our weight forward onto the front leg, sink our weight slightly, and exhale.
There are different types of breathing we can do during the exhale. I want to focus on what Dr. Paul Lam calls "dan tien breathing" (dan tian ? I'm not sure of the proper Pinyin spelling - does anyone know for sure?). You can read more about it here (scroll down to "Dan Tien Breathing").
As you exhale, contract the muscles in the lower abdomen (below the belly button). Focus on relaxing your hips and pay attention to the any feeling in your hips, pelvis, and tailbone. You should feel a gentle tugging that tilts your pelvis and brings your tailbone forward. This straightens your spine and helps direct any force through your legs into the ground. Now you know what it means to tilt your pelvis.
It will be very tiring to do a lot of this type of exercise. It will probably feel as though you have never used those muscles before. You can build on this through practice.