These poses focus on the thumb.
Forward fold : thumb side of wrist
A simple forward fold of the thumb side of the wrist.
Forward fold : thumb, first knuckle
A simple forward fold of the first knuckle of the thumb.
Backward arch : thumb
A back arch which goes almost as much sideways as backwards.
Try to keep your whole hand in the same plane.
Backward arch : wrist and thumb
A composite back arch - covering the wrist and all the thumb knuckles.