Rigid skinning is a really old way of binding characters. I've actually never seen it being used in production. It's best to go with soft skinning if you can. However, if memory serves me right, you should be able to move the vertices of the flexor so that it deforms the way you like. It should then pop to that position when you move the skeleton. Again, it's been a while, so give it a go.
Imagination is more important than knowledge.