The Law of Reversed Effect says that when you consciously try to get something (eg remember something), the more you cannot recall it.
Why is this so? To know the reason, we have to know how our subconscious mind works. The subconscious mind works best when our mind is relaxed.
In the memory example, it shows the conscious attempt to remember things very hard is only impeding the working of the unconscious mind. When you give up thinking, you are actually surrendering to your subconscious mind, letting it takes over the baton.
Then all of a sudden, you remember the thing.