The key to understanding this is the fact that there are TWO motors in the HSD. One is referred to as MG1 and the other is MG2. MG1 is basically a generator, and serves as the starter motor. MG2 is often referred to as the "traction motor" - it powers the wheels, and regenerates electricity during braking.

This is what allows the "motor" to be charging the battery and driving the wheels at the same time. The energy screen is basically combining the motors into one graphically. There are also modes where MG1 is directly supplying electricity to MG2 to drive the wheels rather than going through the battery first.

A very useful site for understanding the Prius' HSD is:

http://www.ecrostech.com/prius/original/PriusFrames.htm

Go to the "Understanding" link on the left. Each subheading has useful info, but there is a LOT of reading!