Point of exercising this technique
The key point of this motion is timing. You should be quick sufficiently, so that you can counterkick him before he drop his kicking foot drop. If you kick him back after he drop his foot, he can continue another kick or move closer or farther. In any case you will fail in your attack.
Then how to control this timing? Rhythme. You should count your opponent's rhythme of motion, and you should find his breath.
"In this manner, the right motion of Taekwondo is not beyond making proper rhythm with both adaptations of you and your opponent to each other."(PPT Ch.14)