On the other path, you take the time to think about your idea. It may even be one of those ideas you come back to time and again. Before long, it appears to be all worked out. It all makes sense, and there are no gaps or flaws. If you still don't start writing you find the idea either fades away as before, or it becomes stagnant. To avoid yet another dead end, let's say you do begin to write down this well-formed thought.
The first sentence comes easily, of course. Even the second presents no problem, but before long you come to an unbridged gap between ideas. Somethings wrong. You were sure that it all fit, so you must have forgotten something. Probably not. The brain has a wonderful ability to ignore missing pieces, to make everything look whole even if it turns out to be an empty shell. Most people pick this time to decide the idea was worthless after all and stop writing. Wrong move. This is the time to start writing. Spotting those gaps and flaws is one of the most important roles writing can play because it opens your ideas for far more interaction.