Get on all fours, with your hands stacked directly under shoulders, and knees under hips. Make sure your back is flat, and tuck your chin slightly so the back of your neck is facing the ceiling. Without rounding your spine, engage your lower abdominals.
Keeping the 90-degree bend in your right knee, slowly lift your leg straight back and up toward the ceiling. Your max height is right before your back starts to arch, or your hips begin to rotate.