Aikido

Aikido is the thinking man's martial art.  The concept of aiki involves blending with an attack to control and neutralize it, rather than opposing it directly.  In this way Aikido's goal is to gain control of a situation, taking minimal damage, and using minimal force to do it.  This has made Aikido a popular form of self defense with people of all sizes.  

Aikido emphasize leverage, soft circular motions and, when performed correctly, allows for the restraint or incapacitation of an assailant (or multiple) with complete control (and, thus, without hurting the attacker--you merely suppress their ability to cause harm).  

History

In the grand scheme of things, Aikido is a baby brother to many traditional martial arts, but demonstrations can illustrate how dynamic it can be.  Created in the early 1900's by Morihei Ueshiba (who is referred to by aikidoka (aikido practicioners) as O'Sensei), Aikido is based largely on several older styles, primarily aiki-jujitsu and even including judo and spear techniques.  

Circles

Many techniques in aikido follow very fluid, often circular curves.  There are a number of 'Circuits of Neutralization' which generally lead an attacker down into a joint-locked position (controlled by leverage applied by the aikidoka), preventing further movement.  

Thus, an attacker's energy is met (blended with), and then directed either away or down (into a Circuit of Neutralization).  

Ki

Very prevalent in Aikido is the concept of Ki.  There is much disagreement about whether ki is a supernatural force (energy) or a harmony of perfect technique (or something in between) but Aikido can yield dramatic results.  It is said that Morihei Ueshiba could cause a person to fall without touching them (simply by holding his hand near their head and moving to 'change the direction of their ki').  

Cautions and Criticisms

Aikido, when performed properly, does not hurt the attacker, but merely controls him (or her).  However, critics point out that because of aikido's training philosophy (traditionally, the partner in practice does not resist the moves), while real-world attackers will resist, and thus aikido training may not prepare one adequately to a real fight.  This is often dealt with by decreasing cooperation and increasing resistance (by more skilled partners, after a sufficient level of control has been reached), and it has also been pointed out that while most martial arts techniques cannot be practiced fully (and must be cut short or simulated to avoid injury to a partner), Aikido techniques can be practiced fully without injury.   

(Soft Styles) Internal Martial Arts

Qi

Qi (also Chi or Ki) means different things to different people, but you'll hear it a lot in the Soft Arts.  Some translate it as breath (of life), or energy.  To many, qi represents vital energy, but whether you beleve in the metaphysical aspects or not, qi can also be taken as that harmonious balance when things just work (the sweet spot, or, to use a hacker term 'The Right Way')--a certain condition when perfect technique creates dynamic results with minimal effort.  Qi.  

Golf

If you've every played golf, you know that you don't get the best hits based on the strength of your muscles.  You'll drive the ball the furthest when your swing is at its best--a smooth fluid motion, just at the right angle, and the club face strikes the ball just right and BOOM!  It's gone.  They say that when you swing correctly, it's the club that does the work. 

This idea is very much like the kind of ideas you'll find in Internal Martial Arts.  

When your technique and your timing are right, you're in harmony with the situation, and your ki is maximized.  It is possible to send an oncoming assailant flying without even touching him--just by moving out of the way at the precise time, your oppenent is left off balance and their own momentum can carry them flying off.  It's less your strength, and more your control. 

Overextension

One common theme in soft styles is getting your opponent to overextend.  The best way to take a hit is to not take it--move out of the way!  When your opponent is lashing out to strike you, but you're no longer there, this can frequently leave them off balance, stretched out, confused/frustrated and/or moving in a direction they had not intended.   

Minimal force can be applied then, as their own body (and gravity) is now working against them. 

Grappling and Joint Locks

Many soft/internal styles emphasize the use of grappling techniques and joint locks to control an opponent.  Okay, so you've stepped out of the way of that kick and you're opponent's got his leg all out there, starting to fall--by using leverage and timing (moving at the right moment), it's possible to incapacitate people much larger and/or stronger than you.  

When in a dynamic situation involving multiple attackers, it is also possible to coordinate the angles and directions you get your opponent to fall in (e.g. sending them out of the fray, or tumbling into their friends to even up the odds).