For example, a corrective blend shape on the elbow may need to affect the shirt as well. This allows sculpting a corrective shape on the elbow, and then automatically creating a matching blend shape on the shirt.
This can also be used to transfer blend shapes between similar meshes. For example, you may have separated head and body meshes with blend shapes on the head, and you want to combine them into a whole body mesh but keep the blend shapes from the head. You can merge the meshes, add a blend shape deformer to the new mesh, then transfer the blend shapes across.
This works by creating a temporary wrap deformer from the body to the shirt, enabling the blend shape, and applying the resulting change to the shirt as a blend shape. The wrap deformer is then deleted, leaving just the new blend shape. This avoids the need to use a live wrap deformer, which can be very slow and can't be exported to other applications.
Create a blendShape deformer on the destination node if one doesn't exist. Be sure that the source and destination meshes are aligned to each another.
Run Deform > Edit Blend Shape > Retarget Blend Shapes. Select the source and destination blendShape nodes, and then select one or more blend shape targets in the list to be retargetted.
Optionally, check "Connect weights to source". If this is selected, the new blend shape targets will be connected to the old ones, so they'll change together.
To update the blend shape later, just run the script again. The existing blend shape target will be updated. Note that the targets must always have the same name, so if you rename one, rename them both.