学习"the sound & style of AmE"的问题

joshma007

昨天开始学 Isaac推荐的 这本教材，有个难点，就是作者所提到的“starting position”。
    原文如下:"Once you can move your tongue into the STARTING POSITION at will, you must learn to produce a downward movement in the rear tongue as you begin making sounds. Follow the instructions on the tape extremely closely as you go through the second half of Lesson #3. You begin with your tongue in the STARTING POSITION and create an "AH" sound using only those muscles in the back of your tongue. This is one of the most important lessons in all of your training. This movement style allows you to create the TONGUE IN THE CORRECT "STARTING POSITION"

1.	Tongue is lying in a concave position (the middle is lower than the sides).
2.	Tip of the tongue is lying just against bottom-front teeth.
3.	Rear tongue is low, not arching up in back. You can easily see the back of the throat between the tongue and palate.

这个starting position 我是可以做到的，但是按照作者的要求，开始位置之后需要“舌后部”下垂，这个要领我试了一天都不行，总是往上，舌中间会更加的凹下去。author建议把fingle tip按在舌中间push down and back这样训练，而我每次这要做都想要吐。不知道Isaac有什么好建议？还有做starting position 舌头要不要用力凹的很厉害呢？

IsaacZ

具体要发哪个音？