Keeping the feet flat on the ground means that the whole of the foot is working. The toes are working and pushing into the ground with just the right amount of force. The ball of the foot is fully connected and not just on one side or in the middle. The whole outside of the foot (lateral longitudinal arch) of the foot, and last but not least, the heel. One can be distributing their weight on the heel, the ball, or the side of the foot, which means the feet are not flat on the ground and the weight is not properly distributed.