Hmmm... you might want to checkout the baseline skillset thread.... because this exercise is the one of the fundamental ways of developing kokyu power. Your "sensei" should have told you that
But a quick pointer. To start, just keep your hands/fists out in front of you and do not move them. Then simply rock your weight forward and backward whilst keeping your hands/fists in place. Focus on the feeling of pressing against the ground, and of transferring your weight forward and back.
DO IT SLOWLY.
Soon you will start to feel how the feet are connected to the hips and how the hands (which are not moving) are displaced as a result. Once you feel comfortable with this, start adding the rowing motions (GENTLY), throwing your fists out as you rock forward, and pulling them back as you rock backward.
AGAIN, DO IT SLOWLY.