Keep your great posture and just lift your heels. Don't change height levels and don't let this move throw you forward or back. The knees still go over the toes.
Your abs are engaged and helping you keep your balance. You might feel your inner thighs and hamstrings kick in and a connection between your heels and sit bones. That is good. This is about getting full engagement of the leg, not just the top of the thigh.
Connect with a sense of vertical line through the middle of your body - your midline. Straighten you legs and raise straight up your midline sending the top your your head toward the ceiling.
Now you are standing tall on the balls of your feet, but not too high. You want to feel the support of the arches of your feet doming under you. You've gone straight up without pitching forward or back.