As far as I can tell envelope forard isn't documented anywhere in the Docusign API and it took quite a bit of digging to figure it out. Here's how it works:
What is document forwarding?
Forwarding creates a copy of a completed envelope which you can send to new recipients as is or add new documents and fields as needed. For a forwarded envelope, you cannot remove any information or signatures on the completed documents, but you can add new recipients, documents, and recipient fields.