The impersonation feature

The impersonation feature lets you access another user’s system as though you are that user. This is a new feature of WebSphere Portal 7.
The impersonation feature lets you view new pages, portlets, and other portal components. Users such as support specialists can use the impersonation feature to find issues and errors.

But pay attention on this two things:

Client side aggregation does not support user impersonation. For this reason, ensure you do not activate client side aggregation on any portal pages where the impersonation portlet is deployed. User impersonation and people awareness: When a user who is enabled for impersonation impersonates other users, the people awareness feature is disabled for the entire session for which that user is authenticated.