This instrument is used for visualizing character's angular momentum.
Angular Momentum is represented as a circle across character's center of mass with a green arrow showing the value of the momentum. Diameter of the circle corresponds to the value of rotational inertia (as this value becomes higher, the circle gets bigger, and vise versa). The red sector of the circle represents character’s divergence from a physically correct position. The greater this divergence is, the bigger the red secor becomes.
Angular Momentum visualizator can be enabled by checking the Angular Momentums option in the Visible menu.