This tutorial demonstrates how to use LaunchDarkly's progressive rollouts in an ExpressJS application for upgrading to a new API version. It covers setting up the required environment and dependencies, creating a flag in LaunchDarkly, configuring a progressive rollout, and monitoring the progress of the rollout. The example used is an API for raccoon facts, with the goal being to switch from Version 0 to Version 1 gradually while monitoring for any potential issues. This method allows developers to control the release pace and mitigate risks associated with new features or updates.