You can usually squeeze a pair of balls pretty tightly without causing much pain. So don't be afraid to grab those suckers and let him know you are in charge. You can squeeze a hard cock just about as hard as you can without causing much pain either. But putting it (or his balls) in a vice will not only be extremely painful, but can cause damage as well. I guess what I am trying to get you to understand is that you should start out slowly and gently. You can gradually increase the intensity to see just where your submissive's pain threshold is. Then you can help him to gradually increase that threshold to whatever level you like. Just remember, it takes time to increase his pain threshold.
Some of you (like me) can be very sadistic when it comes to playing with our submissives. So, please, restrain yourself when dealing with CBT. Always start out gently. You may have seen movies where women have used a riding crop, flogger, or even a cane to strike a submissive's cock and balls. Trust me, one of two things is true when you see this: 1) The Domme is not really hitting the cock or balls, or; 2) The submissive can take a great deal of pain. However, you can strike the cock harder than you can the balls. Just be very careful where you aim.
I will talk about some of the fun things you can do with CBT in future blogs, so watch for them.