The key word is shear.
Imagine an elemental square at the surface of the specimen. Draw it aligned with X and Y axes.
Draw a shear vector vertically up on the right side.
To be in equilibrium there must be an equal and opposite vector on the left side - downward.
That is not the whole story - it just covers forces in the vertical direction.
To stop it twisting in the anti-clockwise direction, it needs balanced forces on the top and bottom edges.
For equilibrium you need a vector to the right on the top edge and to the left on the bottom.
The result is a tendency to pull the square up and right and down and left. It would make the square lozenge shaped.
This gives the concept of principal stresses at 45 deg. to the X Y axes.
Now do it for an elemental cube!
HTH - usually used visual aids when I did it for a living.