Many times people become focused on staying upright so they stay stable and all. In systema we gain our balance from movement. by moving naturally we avoid tension in the body and maintain our freedom. Try to free your mond from being rigidly upright by this drill and make your own too. Stand naturally and start to slide one foot back and forth freely as it is still on the ground but no support is gained by it. To keep your alignment which grants you freedom of motion keep the body in line with the sliding or rocking leg. If you free your hips you will be able to do a lot more with your body with this added measure of movement. For example if someone is trying to lever your neck and his body is somewhat on top of yours you can slide one foot in between his or under his main mass with your body in line with this leg and from inside you simply stand up and he (or she) are on the way down.