Humanoid can be put in certain poses, which can be shared between humanoids. Hand Poses and Facial Expressions are specific poses which use the same implementation. The Humanoid Pose can determine the pose of the whole humanoid.
A humanoid can be set in a specific pose by selecting the pose in the Pose parameter of the Humanoid Control component.
While a humanoid is set in a specific pose, it is no longer possible to change the pose of the Humanoid in the scene view. This is only possible in Edit mode or when no pose has been selected.
A new pose can be created in the project window by right-clicking and selecting Create->Humanoid->Pose.
This pose will be an empty pose which has no effect on the pose of an humanoid.
A pose can be changed by selecting it as the pose for a humanoid and then entering Edit mode by clicking the button at the right:
Now it is possible to change the pose directly in the scene view by dragging or rotating Targets of the humanoids. Edit mode is exited by clicking the Button at the right again: