An echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound. This technique mimics the functions of an echo. Firing off an invisible burst of ki user creates a sort of echo by bouncing it off of the air molecules around them. The echo multiples and bounces back to user as they absorb it to multiply their power. This technique works better in enclosed spaces but skilled users can use it anywhere. The difference between a battle aura and reiatsu is simple: a battle aura is a representation of your life force, your ki, raised to a level so high that even the naked eye can see it when you flare your power, whereas reiatsu is simply a physical force/pressure created by your ki. think of it this way ki is fire a battle aura is merely a representation of that fire while reiatsu is the heat it gives off.
The echo effect of reiatsu allows user to harden their skin, effectively protecting them from attacks as it allows their body to reject negative events by reflecting the force of any attack applied against it back at the attacker. The nature of the attack determines the damage done to attacker blunt force attacks such as punches and kicks reflect back damage differently then more concentrated attacks such as attacks from a blade. Unfortunately this is only done when the concentration of ones ki is higher then that of their attacker.
Stunning: High level of spiritual power can cause those of lower power to be stunned by simply being in the presence of user. Its not uncommon for weaker opponents to get the wind knocked out of them by simply being near a user with a high level of spiritual pressure. The experience is described as feeling like gravity is rapidly increasing around them.
Paralysis: By focusing pressure through their eyes users can disrupt the flow of an opponents ki causing them to loose control of their nervous system.
Damage: Skilled users are able to focus their reiatsu through their limbs or weapons creating powerful shockwaves.damage Those with high ki reserves are said to be able to the surrounding environment as they do so.
Shunkō ( Flash Cry): By reducing the distance that the initial ki burst travels to about an inch away from them, user greatly increases the speed and power of the echo effect creating incredibly dense and pressurized ki around their entire body. The ki is so dense that it vibrates through the ground to the point where user is able to cause great damage to their surroundings. It increases both their offensive and defensive capabilities as they use very dense and high pressured ki to shelter their body, revitalizing it to its max. Since their inside this shell, even if opponent wants to attack them, their attack will never reach. In other words, nothing's going to work on them, nor can anything stop them. The Ki itself can also be controlled and fired at opponents from user’s body, and can be used to create massive explosions.