One of the new features that came with PowerShell v3 was the introduction of “robust” or “persistent” sessions. This is, imho, a huge improvement on the concept of remote sessions who were introduced in PowerShell v2.
In short, before robust sessions, you had to keep the the client, that created the session, open to keep the remote session alive. With robust sessions, the client side can be closed, and you can reconnect to the robust session, even from another client.
This new remote session concept is something that can be used to solve a number of known PowerCLI “issues”.
Siw stands for Server in Waiting. We selected that term since we liked the sound of it, no double entendre should be suspected 🙂