There's another drill that can help you learn proper core activation. Take two light dumbbells and hold them overhead. Now do a high knees march, and as each knee comes up, press up hard with the stance side arm. So left knee up, right arm presses. Try to push the dumbbell through the ceiling. The weight of the dumbbell isn't that important, it's a hip and core exercise, so arm fatigue should not limit you. What this drill teaches you is how to hold your core such that a push on your hip, by your leg rebounding, drives your whole body upwards and forwards.