Left/right. Up/down. Good/evil.
"For every action there is an equal and opposite reaction." Newton's Third Law of Motion
In dance, we apply Newton's Third Law all the time. For instance, when we push our feet against the floor in fifth position, we are able to use that force to jump up for a changement. The more we push, the higher we go. We apply it when we use our arms to help keep us lifted and straight during a turn. And certainly we apply it when we spring our toes off the floor and into a retire position to start the momentum for a pirouette.
But there are all sorts of ways to approach opposition and to use it to our advantage. Here are a few to think about:
1. From the very first exercise at the barre (typically a first position demi-plie), consider the opposition of contraction and release. You must release your gluteus muscles as you lower into bent knees and then contract the inner thigh muscles as you recover to standing. If you have pain in your lower back, you might want to examine whether you are gripping as you try to bend - this could mean you are in a constant state of contraction which actually works against your attempts to get a deeper plie.
2. Also at the barre, every time you move your leg from a closed position to an open one (for instance, a tendu from the fifth position to a la seconde), you need to think about the opposition of standing leg and working leg. It helps me to think of the hip bone of my standing leg pushing back in opposition to the knee of the working leg. You should also be thinking about turning out the top of the standing leg as well as the working leg. This is especially important as you lift the leg off the floor or circle it, as in rond de jambe. Whenever the working leg is moving, you need to think about the standing leg stabilizing you.
3. During grands battements, there is the opposition of inhale and exhale. This works hand-in-hand with contraction and release. In order to maximize the lift of the battement, you want to exhale and think of the leg moving down and away from you, rather than inhaling and gripping the top of the thigh to get it off the floor. The exhalation of breath is calming and reduces anxiety in pirouettes. Next time you turn, try to inhale on the plie and exhale on the releve.