Design
Documenting an idea so that it can be prototyped is only a small piece of the design process. Maintaining proper documentation, revision control, and communication between teams is where design teams can separate themselves from the rest.
While the engineering team will be responsible for managing these operations, it is important for the Product Manager to continuously communicate with the design team to understand the actions taken to form the product under development. In my experience, understanding the design challenges, decisions, and compromises aids in the development of the messaging, sales approach, and differentiation in the marketplace.
As the product gets closer to completion in the lab, testing with customers becomes the next phase of development. Understanding where the product is in design and when to start communicating with potential customers for testing will be an important part to a successful product introduction.