Great question especially since I just covered this on my site. It means that I'm on the right track with things.

There are actually two ways to create that effect.  You can create it using the morph tools found in Poser or you can create it with the Burn and Dodge brush found on 2D graphic editing programs like Photoshop Paintshop Pro and Gimp.

The shadows of her fingers will be made with the Burn tool and the Dodge tool will be used to lighten the skin in order to help fine toon the tension that is placed on the skin. Using the Burn and Dodge maybe faster than using the morph but both work well.  If you are like me and enjoy Postwork then give the Burn and Dodge tool a try.  Make sure the strength of these tools are set between 20% and 30% for a good skin blend.

By the way thanks for posting this question.  I'll definitely share it with those who have taken the workshop. They will be surprised that they actually know the answer to this question.