What is torsional limit?
Torsional limit is the amount of rotational torque that can be applied to a "locked" instrument to the point of breakage (separation). 
Obviously, an instrument should have sufficient strength to be rotated and worked vigorously without separating in the canal.
Smaller instruments (less than size 20) can withstand more rotations without breaking than larger (greater than size 40) instruments.

Ref :Principles and Practice of Endodontics, Richard E Walton, 3rd edition, pg no:156
Amount of apical pressure that can be applied to a file to the point of breakage
The beginning of plastic deformation of the instrument
Amount of rotational torque that can be applied to a “locked” instrument to the point of breakage
Amount of force necessary so that a file does not return to its original shape upon unloading of the force