To the left, I state a conventional definition of the derivative of the function in question. When x is negative as it is on the second line of the derivation, the numerator (|x + dx| - |x|) is -dx because dx is always a positive number. Thus the derivative is -1. On the third line when x is positive, this simplifies to +1.
Plate subduction occurs when two tectonic plates meet at a convergent plate boundary (the plates are crashing into one another) and at least one of the plates is denser than the underlying mantle. If neither plate is denser than the underlying mantle the plates will create a non-volcanic mountain range like the Himalaya.
Instead, subduction occurs as the oceanic plate sinks beneath the continental plate. When two plates carrying continental crust collide, subduction does not take place. Neither piece of crust is dense enough to sink very far into the mantle. Instead, the collision squeezes the crust into mighty mountain ranges.