The best way I can think of to illustrate this to a student is to place an object (pen) on a piece of paper on a flat surface (table).
The pen is the aircraft, the paper is the "parcel" of air the aircraft is flying through.
I move them both across the table with the pen move faster than the paper, thus representing what is happening when flying in a moving parcel of air.
I then stop the movement of both the pen a paper, letting go of the pen, I then tell the student to watch what happens if the speed of the "parcel" of air changes. I jerk the paper and the pen gets "left behind".
I then explain this is what happens to an aircraft when the parcel of air in which it's flying changes direction or speed. The result is manifested in a momentary increase or decrease in IAS while the aircraft catches up.