Bending forward is not spinal flexion
Two people can arrive at the same shape with opposite spines.
Describe a pose twice: once by where it goes in space and once by what the spine is doing.
Forward bend, backbend, side bend and twist are descriptions of direction in space. Flexion, extension, lateral flexion, rotation and axial extension are descriptions of what the spine is doing.
They are often mixed up, because in some poses they happen to agree. In Chair Pose the body folds forward in space while the spine keeps its curves.
In a rounded seated fold the body also goes forward, and this time the spine really is flexed. Same direction, different spine.
Once you hold the two descriptions apart, a great many arguments about poses turn out to be two people describing different things.
Forward in space, flexion in the spine
The direction the torso travels and the shape of the spine are separate observations.
This is a simplified learning diagram. It is not a reviewed anatomical illustration.
Chair Pose
Open Chair Pose and read both chips, then open Forward Fold and read both chips. Say out loud where the two descriptions agree and where they part company.
From sitting, tip your whole trunk forward from the hips, keeping the spine long. Then come up and round your spine forward instead.
Notice: Both moved you forward in space. Only one of them was spinal flexion.
This is noticing, not instruction. Move as little as you like, stop at any point, and watching instead of moving is a complete way to take part.
Can you name a pose where the two descriptions disagree?
Pause before opening Practice. Say the distinction in your own words.
Which description are you actually making: the direction, or the spine?
Breath and movement material here is study language, not instruction or therapy. Every invitation is optional: move as little as you like, stop at any point, and watching instead of moving is a complete way to take part. Do not prescribe breath holds, retention or bandha practice.