I ride a horse that accelerate in that situation, each time he does I make circles and I disengage hindquaters. This way I make the right thing easy and the wrong behaviour hard and uncomfortable. It works well with him.
I agree with Lauren, all your extension neck session will depend on your seat! You also can ask with the voice, and highlights your seat's asks with also your back :)
Adjust your position so that your seat is slightly more forward in the saddle; however, don’t allow your upper body to tip forward. Continue sitting tall in your saddle. I think that the secret is in the seat