  We show that rotation of the magnetic field off the plane of twin boundaries
(TB's) induces transition of an ordered vortex solid phase to a disordered one.
This transition arises due to appearance of transverse deformations of vortex
lines near the TB's, $u_{t,TB}$, the amplitude of which satisfies the Lindemann
criteria, $u_{t,rp}=c_La_0$. This order-disorder transition is accompanied by
an increase in the depinning current, by crossover from an elastic creep to
plastic one, and by appearance of the $S$-shaped voltage-current
characteristics.
