Kiai eventually becomes a part of you. After a while, you hear yelling and you automatically think "KIAI! MORE!!". Personally, when I go to watch my friend's Karate classes, I find their kiai is... Relatively pathetic compared to kendo's. I guess it's the mentality of each individual martial art - kendo is 50% speed, 50% accuracy, >9000% kiai. Fumikomi hurts at first, but then you just start to get used to it. All of the nerves on my feet are all dead - I can't feel any pain at all anymore. [I had massive blisters at the beginning, now they're armour more than anything.]