I read that koan practice is meant to create doubt.
But what happens when the koan has been passed? Does doubt end, or begin again, or get literally "shattered"?
Does the practitioner experience a moment of hope?
It is not just any doubt, it is a highly perplexing doubt, a sense that the entire world is pulling your leg.
When the doubt gets shattered there is a return to natural clarity, free of false hope.
But because the koan-induced satori is more often not the complete enlightenment, there is still some lingering seeds of hope in the background, which eventually come back to life and become the new round of longing, aversion, and confusion.
From my personal experience, dropping hope is a good thing - but not dropping the koan type of perplexing doubt. This doubt is like the smell that gets you to the cheese.
Andrei's answer is pretty spot on, but I think you need a little background on koan practice to really get the gist of the kind of doubt it's referring to. Say you are given the Mu koan. You might sit with that for a couple of months. During an interview with your teacher, he might ask what mu is or to show it to him. You might say "it's emptiness". Ding! He rings the bell. Wrong answer. You go back to sitting. You go to him again and say something like "thoughts stopping". Ding! Back you go. "No mind?" Ding! "Everything is one." Ding! "The cypress in the garden." Ding, ding, ding!
The whole time this is going on, you are getting frustrated. Your hope is getting crushed. It seems like everything you say is wrong. All your theories are dashed. Even what you take to be subtle observations aren't the answer your teacher is looking for. Your mind has no refuge. You have nothing to hold onto. It's all not this, not this! That is great doubt. And it's visceral. Mumon even likens it to swallowing a hot ball of iron.
But if you practice, you will eventually come tottering into a place where insight is possible. You might say something to your teacher, something you take to be completely outlandish or that you don't even understand yourself, and he'll pass you on your koan. On some occasions, you may even get a pretty clear, momentary glimpse of the blue sky through the clouds. But most of the time, and I don't mean to disappoint, but even after "answering" your koan, you'll walk away feeling just as confused as ever. The doubt comes slamming back down on your head and back to the cushion you go...just this time with a different koan.
Koans are like holes knocked into a wall with a sledgehammer. Every one that you answer does a little damage. Some might even let a little light through. But over time, if you smack it enough, it will eventually fall over. Only then is anything really shattered. It's best not to have hope. Hope is a temporary refuge. Better to come at that wall with the full force of your doubt.
The answer goes beyond words ,doubt does not exist anymore or neither does hope or any cause or condition only when we drop off body and mind do we see the true nature of ourselves reallity of oneness that you become everything around you and it becomes you.Mind is only your own idea or concept that there is no mind that there is nothing yet you are everything but you can't explain it in words this can take one moment to see any a lifetime of sitting with mu to get the satori experince in zen .Going beyond dualisim that there is no self that the dharmakaya is you or you are the Buddha realised so is everything else around you