An example of an "external brain" is:

Organize things

Explanation:

It is the duty and power of external brain to Organize ideas. Per day we may have plenty of works to be done. So we start thinking about one by one and finally we try to sort out and organize how to perform, when to do and what is the first and last task etc.

It is the external brain which is helping us to figure it out. But there are certain times where we dump things together, especially when there is an very important like marriage of our family member, where they give lots of responsibilities.

At that time, we have very limited time to work out things and we dump our external brain.