Hi Jan
Can i just add some things to what Don has already said.
NEVER stick your rear end out while dancing.
Here's a little exercise to show what's meant by having this nice backward poise which you see so often in the good dancers, and yet having the weight forward.
Stand in a good upright position, and then take your weight onto your toes by standing on your toes. By being on your toes, your weight must be forward over the standing foot.
Now, staying on your toes, stretch your upper body up and slightly back in the 'classic' lady's dancing position (don't overdo it). Now you're poised back, but your weight is still forward, since you're balanced on your toes. You will notice that your hips move forward (into your partner, if he's there) this is inevitable because of the laws of physics, which you will never be able to defy!! But it's a good thing, because it keeps good contact with your partner. As Don says, you must keep this poise and balance as you move, but practise it statically first.
If you're concerned about the flexibility of your upper body, you may find the 'cobra' exercise of great benefit for making your upper back very flexible. Do it 3 times a week, and after a couple of months, you will notice a great improvement.
Good luck

Since the Lady's head weight is carried over her left heal, I would think she would lean slightly back and to her left side. Dave

Thanks so much everybody... I have used all of these exercises and after twisting my body in a million different postions I am beginning to feel what everybody has written!

To answer a previous question.. Yes my questions on frame and posture are for international standard.

It is easiest for me to understand my knee going forward while dancing backwards in the foxtrot especially in the feather and three step. I still have not quite figured this out in the waltz and quickstep just yet.

Again, thanks for all the help

Absolutely right! Lady should maintain such backward poise throughout the dance.( Classic lady position . With rotating the left side region from hip to left armpit , put spine and head over the left heel...) Detailed explanation seen in the page 11 of A Technique of Advanced Standard Ballroom Figures writen by Geoffrey Hearn.
But I do not understand 1.why we can get vertical line through central line while we sudden stop following a quick run in the place. Yes, there is some central muscles tension when we run quick, is the musles strenght enough for extend body line ?.How do the central muscles work in ballroom dance exactly? ( Please give me more teaching tip about the working muscles .How to gather the energy inside our body ? )2. why level belt means vertical to floor ??
A board vertical to floor may have lines level or not level.
Thanks.

Yichen-1. The lady's poise does not stay the same throughout the dance for example, in chasses and telemark and certain other places her poise is more upright and in certain lines her poise is extended further back. Dave.

Dear Dave,
Yes, you gave me very good examples where the poise changed . Poise should be changing when needed. Thank you very much.
Recently, I had more time ( no lessons transiently )to consider some fundamental basics, such as posture , poise, vertical line, swing.
Although I did not make clear the whole meaning of "central muscle position when quick running " mentioned in page 11, (I prefered to use muscles tension to replace muscle position . )I felt the mentioned central muscles tension is helpful in posture and movement . I got more freedom from flexible spine ( Maybe I overused the core muscles I thought it may be why Geoffrey emphasized no contraction. )
Last night , I accidentally found out the reason about level belt:our body is not a board ; pelvis will be in its mid position while the belt is level. I am trying to maitain level belt in some figures such as progressive backward, forward, pivot.
It seems getting more upright and flexible lumbar spine and relaxed chest now which let the swing easier.through swing , the weight cross my standing foot and floor clearer and faster and safe energy. Body moves as one block. It is different with my previous experience. I feel surprise, excited and not sure wheather there is misunderstanding. I want to try, to feel...