Hi Camilla -
Two things -
1) make sure you are not holding so much tension when engaging your low abs that you create tension elsewhere. You need to hold enough to stabilize your pelvis (so it doesn’t rock up and down and you don’t bend at the waist when you lean), and as you lean to go faster, you just increase that engagement enough to keep your column in line from the shoulders to the ankles.
2) Imagine you have a belt on just above your pelvis. Breathe over that belt, allowing your belly to spill over the belt a little. To get a better grip on breathing while engaging, try the floor exercise found in the posture section of Chapter 4 of the Chi Running book. While laying on your back, knees bent, press the low back to the ground. As you slowly walk one foot away, purse your lips as you breathe out through your mouth. Put your hands over your stomach and feel the rising and falling of the breath as you keep your low abs engaged.
Let us know if that helps!
Certified Chi Running and Chi Walking Master Instructor