It looks like in the current pipeline, in Rig-Pose node, we control the joints rather than bones.
However, if we look at this situation:
We want to bend character's upper body back. Obviously, the characteris upper body is where all polys above the pelvis node.
In 3DSMAX/MAYA, we can directly rotate the Pelvis-to-Spline1 bone to achieve this upper body bending.
However, in Rig-Pose node here, we cannot do that directly cause we cannot select bones, we can only select joints.

For example, in this picture

The way I bend the upper body is to: Translate the spine1 joint backwards and downwards and then rotate it. That's not very enjoyable cause I cannot guarantee a persistent bone/segment length. I might add/reduce length of bones.

In other words, If I want to do this by rotating the pelvis node, how do I prevent FK chain to the foor joints?

But anyway, directly rotating bones/segment looks much cleaner. We can select bone/segment in skeleton node and rotate(despite it actully does not rotate it), but we cannot do that in rig-pose node.
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Not sure I 100% understand, but I usually approach it two different ways. The problem, I think, is that you want to have 2 different children of the pelvis joint-- the upper legs and the spine. But you can't have two opposite children. So instead you have to split the legs from the spine.

1. Separate the base of the spine from the pelvis by creating a second joint close to the pelvis, but constrainted to it. Translating the pelvis will still move the spine around, but you can then rotate the spine without affecting the legs.

2. Of course, you can also use an IK spline set up to control the bend of the spine. I usually find that easier to animate and more natural. But I'm guessing you have your reasons for wanting a strictly FK set up.