What is the technique used during the activities definition process that breaks down the "work units" in a work package into smaller and smaller chunks to identify what activities are required to complete that "work chunk"?

In project management this technique is called "Logical Dependency Technique"

Explanation:

A dependency highlights the logical relationship between two or more activities or task to be carried out in project management

There are basically four types of dependencies

1. Start to start

2. Finish to start

3. Start to finish

4. Finish to finish