Understanding this simple concept with will improve many of your brazilian jiu jitsu moves and techniques.
When your limbs are close to the center of your body they are in a far stronger position than when they are in extended away from it.
Try this simple test:
Hold something weighing approximately 10 lbs close to (but not touching) your solar plexus with one hand. Note the sensation of its weight. Now extend your arm parallel to the floor away from you and notice how much heaver the object feels.
In a jiu-jitsu situation this is especially evident. Not only are your legs and arms much more vulnerable to joint attacks when away from your center, but their strength is greatly diminished as well.
Using this principle, it becomes evident that if we are able to maneuver all of our opponent’s limbs away from their center that we are placing him in an extremely weak position. This process of ‘separation’ can be applied in many grappling positions to undermine your opponent and allow you to exploit the situation to your fullest advantage.
Conversely, by keeping your limbs close to your center you almost always improve your posture in defensive positions.