Knowing the principles of jiu jitsu doesn't mean you will understand them. Understanding them doesn't mean you can apply them.
Learn to Stand
By controlling the way you stand you control the parts of your body that present a target for your opponent. A good stance makes the body narrower by turning sideways on. It shields the more vulnerable parts of your body behind the bones of your arms. Turns your hips so that the groin doesn't present an easy target. The elbow shields your kidneys and floating ribs.
Ma-ai - "Harmony of space"
If you are too close to your attacker you will not be able to react quickly enough to their attack. You will also find it more difficult to see the whole of their body, every part of which could pose a threat.
Kuzushi: Balance Broken
In order to throw someone you must first break their balance. Being off balance doesn't just apply to a physical state, it can apply to a state of mind. Weakeners, distractions and diversions can all unbalance an opponent, by drawing their attention away from their control over your arm, your face or their knife.
Minimum Force Maximum Effect
A small amount of force applied in the correct way is sufficient to immobilise an attacker. Applying this force requires skill and speed, but it does not require strength. A good rule of thumb in jiu jitsu is that if it took effort it wasn't as good as it could be.
Control the Head, Control the Body
The apparatus that allows us to balance is located in the inner ear. By controlling the position of your opponents head you gain control over their balance. As a child we learn to balance naturally by responding to a state of unbalance. Adjusting balance is a calculation that we make automatically. We are even able to make adjustments for heavy loads we may carry. We find it harder to cope when our view of the horizon is affected. If our eyes are shut or we are forced to look directly up we respond more slowly to changes in balance. We use these effects in jiu jitsu. By shielding the opponents eyes or by tipping their head upwards we slow their response to being off balance. Tipping the head can be done in a number of ways, from pulling their hair, to the use of nerve points around the head, weak points such as the eyes, using "handles" such as the chin and nose, or even striking the head. You can even influence it without touching it by suggesting an attack on their eyes for instance. Move the head and the body will follow.
Using Body Weight
A woman weighing 60 Kg is certainly at a disadvantage against a man weighing 100 Kg if she is involved in a pushing match. His greater mass can generate greater force. If he grabs her wrist she will find it difficult to pull away from him, but however strong he may be, it would not be easy to lift 60 Kg. Harder still to lift the weight with one arm. If he's made to lift it at arm's length it requires a great deal of strength. This principle is used in many techniques in jiu jitsu to gain control over someone who is bigger and stronger.
Look in the mirror and draw an imaginary line down the centre of your body. Now imagine a zone 6 inches either side of that line. This is the main area of your body to defend, and the main area of your opponent to attack. People often exaggerate their movement by stepping further than they should or extending a block further than they need to. Looking at the centre line it is clear that in order to defend the main area of your body you do not need to move far from the centre line.
Blocking a punch or kick in some styles requires a great deal of force. The block is achieved by striking the attacking limb to deflect it. Jiu jitsu employs circular blocking that deflects the punch gently, and only a little. Hard blocking requires more force to overcome inertia and it creates momentum in the attacking limb so that you lose contact with the attacker , and lose influence over their body. Soft blocking allows you to gain control over an arm or a leg. This allows you to gain control of the attackers body. It also avoids the need for excessive force to deflect the limb, and makes it less likely that the block will miss by being too quick. The circular movement of a block follows the arm from its origin at the shoulder. Any force must derive from this point and the shoulder doesn't move as quickly as the fist. What applies to a punch or kick applies to ways of attacking. By moving around an attack You can redirect the force of your attacker and use their energy against them.
Bend Your Knees
One way of forcing your opponent to work harder when holding on to you is simply to bend your knees. This simple action may force them to lean forward, break their posture and make their position weaker. Bending your knees achieves other things. It means that you present a smaller target for the attacker, makes it easier to deal with kicks, and enables you to make better use of your body weight. Pushing a heavy object like a car requires that you use your weight effectively so you bend your knees and lean into it naturally. However, when given a movement to perform that isn't natural to them people are usually stiff and awkward, yet moving an attacker is like pushing a car, you need to bend your knees. It also works the other way, bending your knees makes it harder for your opponent to push you around. If they are forced to apply more strength lifting you off the ground they have less to force you backward. In order to throw someone, particularly in hip throws, it is important to lower your centre of gravity. Generally throwing is easier if your centre of gravity is below theirs, this way most of the force applied to them is upward. Throws generally involve rotation around your centre of gravity. When performing a hip throw: you bend your knees, use your body weight to pull the attacker towards you, break his balance, and guide his momentum and attacking force around you, in a circular motion that leads him to hit the floor...hard.