When you spend way too long scouting, when you are too scared. Some people can really push their limits while still remaining in their comfort zone. Others can be forced out of their comfort zone very easily. I know I'm in my comfort zone if I can look at a line and visualize myself hitting it bang on. When you suddenly stop hoping the bigger drops are around the next corner.
People are different.
Some freeze up. I get ultra focused. I tend to zone-out everyone and everything except the task at hand. That is how I know I'm pushing myself. Some people fall apart.