The flat top breakout is very similar to the bull and bear flag pattern you can find here. You have the same trend, followed by consolidation period and then a continuation of the trend.
The main difference will be during the consolidation period. In the flat top breakout instead of the consolidation having a gentle slope in the opposite direction as the pole, there is no pull back and the consolidation takes place within a few pips of the new high. To go along with this time of consolidation you will see very low volume compared to the initial trend.
These patterns have some of the most powerful breakouts and should be immediately entered when you first find them as the price is not likely to retrace back to the breakout price.
The below chart is gold the beginning of the year where the trend has moved in a narrow upward trend before hitting the flat top. The top is tested several times again within a narrow range before it breaks to the upside. As the top had been tested several times it gives you a great entry point just above to ride the breakout. Due to the length of the sideways movement, the breakout is stronger than you would see with a shorter consolidation period.