Two runners: a sprinter and a marathon runner. The sprinter is first to change pace. When he overtakes the marathon runner, you can see the race has sped up.
Switch on SMA 20 and SMA 50 in the simulator. While the fast one is above the slow one, buyers are stronger. A crossover shows that the balance of power has changed. The most famous crossover is of the 50- and 200-candle averages on a chart where one candle is one day (in the simulator that's the 1Y scale). It's called the “golden cross” when the fast one goes up, and the “death cross” when it goes down.