Let's take a look at a specific example that came up in my class last night so I can hopefully make my point clear. In the Chito Ryu kata Ni Sei Shi there is a particular spot where you retreat by pulling your front leg back and to the centre, block with the left hand and strike with the right hand. You can watch the technique here: http://www.youtube.com/watch?v=GYzuAnsldzQ at the 1:29 mark in the video. You'll see that the individual on the right slides his left foot to the centre and blocks (shuto uke) the attacker's punch with his left hand. There are a number of points to this part of the kata but I'm going to examine just the block for this particular discussion. Shuto-uke.
When it is done correctly, it is more of a redirection which steers the attackers punch down the side of your body thereby preventing you from getting hit and setting the opponent up for the counter attack - a right-handed shuto uchi which you see at 1:32 in the video. If the block (shuto-uke) is too high or too low then you will miss the attacker's punch and get hit. If you do not block with enough force then the punch will drive through the block and you will get hit. If you block with too much force then the block (shuto-uke) will push the attacker's hand too far and the attacker, realizing that his punch has been pushed off course, will use the rotational energy you have provided them with to hit you with their left hand.
The correct method is to block with just enough force to keep your opponent's energy moving forward and to prevent you from getting hit. By keeping them moving forward you actually allow them to step into the counter attack making it even stronger. Also, it takes them longer to realize that their attack was not effective since they continue to move forward with the punch. Without this realization there is no need to attack you with their other hand as they believe their first punch will do the trick
By gaining this understanding of the technique you are better able to practice the bunkai with your partner and you have a better understanding of how to perform it in your kata. So, take the time to understand each and every technique in a kata and what that technique is trying to accomplish. Your kata, bunkai, and kumite skills will improve with understanding.