Correcting the wrist – flexion
To pose the mesh into the troubled poses, I will only be using the FK controls. This will allow me to isolate which rotational axis of a given joint creates the pose and can therefore be used to drive the corrective fix. In the case of the wrist, it's mainly when I create flexion and extension (Rotate Z), that we seem to lose volume in the region. So start by rotating the l_palm_ctrl by -70 degrees on the Z-axis and then duplicate the mesh by hitting Ctrl+D with the character in that pose. With the duplicated mesh selected, highlight all the translate, rotate and scale attributes, hold down the RMB and go Unlock Selected. You should now be able to translate the duplicated mesh back so it does not sit on top of the original mesh.
Now push and pull the vertices to correct the pose. I tend to use a combination of simply moving vertices around, mainly with Soft Selection active (hit B on the keyboard to activate/deactivate) or by using the Sculpt Geometry Tool which you will find under Mesh. Setting the Operation to Relax or Smooth on the Sculpt Geometry Tool is a great way to even out or smooth the faces in a particular region.
Once you are happy with the corrective fix, select in this order: the corrective shape fix, then the original mesh and run the 'BSpiritCorrectiveShape' command. Once it has performed its magic, a new geometry should be created and this will be the model we will apply as a blend shape. Delete the mesh we originally duplicated to create the corrective fix and then rename the newly created mesh l_wrist_rZm70. The reason I've called it what I have is because this will inform me that it is a corrective problem of the left wrist (l_wrist), I have rotated it on the Z axis (rZ) and the rotational value is -70 (m70). You can come up with your own naming convention if you wish but I find this works for me. Now with l_wrist _rZm70 selected, hit Shift+P to un-parent it from its current location in the hierarchy.
The original wrist pose and using Soft Selection to correct the wrist
Mirror the corrective shapes
To create the corrective equivalent for the right palm, duplicate l_wrist _rZm70 and rename it r_wrist _rZm70. Run the abSymMesh.mel script to load up the GUI which we will use to mirror the corrective pose. If you have not already, select the male_geo (make sure it is back in the default pose) and click the Select Base Geometry button on the abSymMesh GUI. Once it has done its magic, select r_wrist _rZm70 and hit the Flip Selected button. You should now have 2 corrective fixes, one for each wrist.
Using the abSymMesh tool to create the right-side corrective shape
Applying the blend shapes
Select, in this order, the 2 corrective shape fixes and then the male_geo (the default mesh). It is important that you select the object that you would like to apply the blend shapes onto last. With all 3 meshes selected, go Create Deformers > Blend Shape (Options). Call the BlendShape node 'correctives' and then switch to the Advanced tab. It is important that we set the Deformation order to either Front of chain or Parallel (I've opted for Front of chain) as we will want the blend shapes to perform before the skinCluster does its work. Once you've set those options, hit Create. If you now select the male_geo, you will now find a 'correctives' node under the INPUTS in the Channel Box or in the Attribute Editor. Open that node up and you will have a 0 to 1 attribute to drive each corrective shape. With the corrective meshes still in the scene, you can also update the fixes should you need to, and this will be propagated down to the 'corrective' blend shape node. It will be a bit much to ask the animator to open up the 'corrective' blend shape node and animate the corrective shapes, so later on we will hook them up to be driven by Set Driven Keys.
Applying the corrective blend shapes to our skinned mesh
Correcting the wrist – extension
Moving on, let's create a corrective shape for the extension of the wrist. Select l_palm_ctrl and set the Rotate Z to 100, duplicate the mesh and unlock the transform attributes for the duplicated mesh. Translate the duplicated mesh back and, using any tools you wish, add more volume to the wrist. Here again I used a combination of the Move tool with Soft Selection turned on and the Sculpt Geometry Tool.
When you are happy with the fix, select in this order; the duplicated mesh, the default mesh and run the 'BSpiritCorrectiveShape' command. Delete the previously duplicated mesh and rename the new shape l_wrist_rZ_p100. Now duplicate l_wrist_rZ_p100 and rename it to r_wrist_rZ_p100. Using the abSymMesh tool once more, select r_wrist_rZ_p100 and hit Flip Selected.
Using the Sculpt Geometry Tool to work on the wrist