twistY, jointY, twistX, jointX, twistZ, and jointZ are affectors. They tell the joint to affect not only the moved item, but th item it's attached to. Same with the simialrly named ones with the other part in the name (i.e. lForeArm_twistX and such). If you remove them you get inorganic behaviour. For instance, if you were making a posable door you would place the door centre along a line betwen the middles of the hinges and the endpoint out at the edge of the door in line with the centre, remove the xrot and zrot to keep it from bending except the way it's supposed to, remove all affectors (what things you asked about) from both the hinge mount and the door so that the door moving doesn't bend the hinges, remove the transes from the door part so it doesn't walk away form its parent, set the limits so that the door doesn't wrap back through its own hinges, and then set forceLimits to 4 (or I guess 1) so that the user can't screw the door up because they didn't turn limits on. Oh, and there's no real use for the weld at that point either.