  We study the spherically symmetric collapsing star in terms of dynamical
instability. We take the framework of extended teleparallel gravity with
non-diagonal tetrad, power-law form of model presenting torsion and matter
distribution as non-dissipative anisotropic fluid. The vanishing shear scalar
condition is adopted to search the insights of collapsing star. We apply first
order linear perturbation scheme to metric, matter and $f(T)$ functions. The
dynamical equations are formulated under this perturbation scheme to develop
collapsing equation for finding dynamical instability limits in two regimes
such as Newtonian and post-Newtonian. We obtain constraint free solution of
perturbed time dependent part with the help of vanishing shear scalar. The
adiabatic index exhibits the instability ranges through second dynamical
equation which depend on physical quantities such as density, pressure
components, perturbed parts of symmetry of star, etc. We also develop some
constraints on positivity of these quantities and obtain instability ranges to
satisfy the dynamical instability condition.
