The best way to understand how this pattern works is to study collection of charts with real life examples.
Expanded Flat is a flat consolidation that typically happens after a strong rally or decline. What makes it tricky is a corrective wave b up that moves in direction of the main trend and manages to make a new higher high over the preceding impulsive wave. In addition, the final wave c down is so strong so it manages to break under the low of the first leg down in wave a down.
The best way to understand how this pattern works is to study collection of charts with real life examples.