public static class SixDofJoint
From bullet manual:
This generic constraint can emulate a variety of standard constraints,
by configuring each of the 6 degrees of freedom (dof).
The first 3 dof axis are linear axis, which represent translation of rigidbodies,
and the latter 3 dof axis represent the angular motion. Each axis can be either locked,
free or limited. On construction of a new btGeneric6DofConstraint, all axis are locked.
Afterwards the axis can be reconfigured. Note that several combinations that
include free and/or limited angular degrees of freedom are undefined.