It reduces the strength of the Joint Controlled Magnets effect. Zero gives full strength, minus 1 gives zero strength.

What's that? You don't know what JCMs are? Basically, they are built-in magnets that operate when you move a joint away from its zero position. For example, lift an arm, and V4's breast lifts up and stretches. It's a JCM doing that.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)