General Problem of Dollyeo Chagi
Compare two examples of basic Dollyeo Chagis in the movie. You should be able to see that the former is good and the latter is bad, and to see in what point they are.
The former kicker begins his kick right away as he hear sign, but the latter turn his supporting leg first and begin his kick. The difference is subtle, but its result is serious. In good case you can succeed in your attack, but in bad case you will fail.
Remember, "Simple motion is in an instant. It precedes the opponent's reaction because it doesn't hurry yet connects necessary movements only. Therefore, even though you move slow he cannot defend it."(3JKY, Ch.3-6)
To See this prroblem from Side View : Click Here