What is it?
Shadowing is a kind of structured observation which generally occurs users authentic environment. It is based on monitoring the user while using something. You can access data that may not be obtained by other methods.
How can I use it?
Firstly you have to identify the activities which you want to observe. Then you need to create a trust between you and user.
When can I use it?
If you need insights into how the user does a job or uses something, you can use shadowing.