Jaki

Jyaki (also called youki), often translated as "youkai energy", is commonly the "magical energy" around a youkai, sort of like a malicious, demonic aura.

the stronger the demon, the stronger his jyaki is. normaly the youkai energy can't be seen by anyone but by using the kaze no kizu an later in the manga the dragon scalled tetsaiga inuyasha is able to see this jyaki and use it for defeating a demon.

attacks.
inuyasha's sword, the tetsaiga, uses the foes jyaki in most its attacks.


 * kaze no kizu: this attack actually cut's the opponents jyaki. that is why it's transelated as 'the cutting wind'' (or wind scar.)
 * bakurayuu: (transelated as: the backlash wave) when a youkai does an attack, he or she sends a great amount of jyaki your way, with the backlash wave Inuyahsa reverses that energy, sending it back to the opponent.
 * dragon scalled tetsaiga: is a technique Inuyasha learns later in the manga. with this atack, he destroy's the most important jyaki of a demon, instantly killing it.

others:
here are some other things that use the jyaki of a youkai: