The Prequel videos are not about sails specifically but instead cover aspects of node.js and its related ecosystem that I found relevant to understanding sails itself.

The Prequel to Sails.js and Node – Part 1: Understanding and creating a node-based HTTP server.

The Prequel to Sails.js and Node – Part 2: Creating a module and parsing a URL in node.js.

The Prequel to Sails.js and Node – Part 3: Creating a router and controller from scratch.