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The KBSJoint is a type of Karma constraint. Used in conjunction with actions using PHYS_Karma, Karma constraints can be used to make more realistic and more believable simulations. See the base KConstraint page for more information on basic constraint properties and how to set them up.
The KBSJoint simulates a "ball and socket" style of joint. The two actors constrained by this actor will be able to pivot around the point determined by the location of this actor. More technically speaking, the actors constrained are not able to change their distance from this constraint or their facing relative to it, though they may rotate freely around the constraint actor.
This type of constraint has no special parameters to set and is pretty much "plug and play." Follow the directions on the KConstraint page to find how to set up a constraint between two actors, or between an actor and the world.