With a Kettlebell in the rack position take a large step forward. Your goal is to lunge as deep as possible keep your front knee from shooting past your toes while maintaining an upright torso. Stand up by driving your front heel into the ground.
Pushing off of the ball of the foot means you did not step forward far enough.
Be sure that the weight is not augmenting the movement, if it is the weight is too heavy or you are not ready for a loaded lunge.