Yusuke has the ability to infuse different parts of his body with his spirit energy, which is basically his aura. It's possibly to channel it through any part of his body he chooses, but Yusuke is partial to firing it from his index finger, forming a gun. This is his trademark move, and also why the majority of his fan art shows him making a gun shape with his hand.
Exhibit
one and
two. Video examples can be found
here. The down side to this is that if he expends all of his aura, his ability to use his spirit gun will vanish. One several occasions in canon, he has dipped into his reserves and used energy devoted to keeping him alive, and nearly died in the process. He will only take those measures in a life or death situation. (Not necessarily his own life; he'll risk more for his friends' safety than his own.)
The spirit gun has two "settings," stun and kill. When fighting a human, or sparring, Yusuke automatically uses the stun setting. He is not looking to kill anyone, so while his opponent will feel the blasts, they won't sustain any sort of life-threatening injury. The kill setting is something Yusuke reserves for anyone frighteningly stronger than him, demons, and anyone is looking to harm his friends. If he scores a direct hit with the kill setting (ie, chest, head, any of the vitals) then congratulations, you're dead.
For the City, this has been toned down considerably. He’s limited to four shots at full power, as opposed to the nearly unlimited amount he has at the end of canon. It still packs a wallop, but if he uses up all four shots at full power, he’s done. What this means is that, unless I state otherwise, he won’t be using his full power shots. He’ll be splitting his energy into smaller, less devastating shots.
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