Contrast is commonly used for visuals and sometimes for music but you can also use it for game difficulty. The usual pattern in nature is slow rises and sharp falls.
- The stock market tends to slowly go up and occasionally drop dramatically.
- When you build a sand castle on the beach, you slowly make it larger, and then suddenly a wave destroys it all.
- Forests grow slowly, and once in a while a fire will wipe out large sections of forest.
- Stress slowly builds up in rock faults, and then an earthquake will suddenly release a large amount of stress.
In games we see this same pattern. Think of how many games you've seen that start off slow, build up, and then have a “boss” fight, followed by the relative calm of the next area. The difficulty or excitement might look a lot like a stock market plot.
One major difference between games and the patterns in nature is that you can't always predict the falls in nature. I've seen some games where a boss is unexpectedly followed by another boss, but usually you roughly know what's going to happen after a boss fight. In games where encounters are dynamically generated, it might be interesting for the encounters to follow a pattern like those seen in nature. More randomness with slow increases and sudden drops could make games more exciting.