From http://www.albanypowerexchange.com/BDSMinfo/limits.htm
Limits
In BDSM, limits are any kind of boundary or restriction placed on one or both partners. They can be physical (e.g., a bad knee or back might make certain types of bondage unsafe), mental or emotional (e.g., something that triggers a phobia or recalls a past trauma), or experiential (anything that one partner just isn't ready for yet).
Finding your Limits
Novice players might not fully understand the concept of limits, and they might not yet know where their limits lie. For a truly novice player, one of the goals of your scene must be to carefully and systematically explore your potential and identify where those limits lie. Don't underestimate the power of a first experience! By pushing limits too hard, a novice might be completely turned off to the whole thing, and never experience the joy of a well crafted, mutually fulfilling scene.
For the experienced player who is running a scene with a novice, it's analagous to the care of a child. People very new to the scene are usually completely ignorant -- and I don't mean unintelligent, just uneducated. They don't know what questions to ask you, and they don't know what information to tell you. As the experienced partner, accept responsibility for your partner, and take a little extra time to educate and inform him or her. Novices are almost always nervous, sometimes frightened, and a little TLC goes a long way. Remember how you felt during your first scenes. No matter how long we've been in the Scene, we all had a first time.
Expanding Limits
For most of us, limits are not set in stone. They tend to change over time, and can vary due to the circumstances of a particular scene as well. You might be willing to go a little farther with a particular individual because you know and trust them well. This is entirely natural, and you should expect it. A shifting pain threshhold is perhaps the most obvious example of a moving limit.
Limits can be moved by conscious choice, as well. Some of us, particularly those who have been in the scene for a while, become dissatisfied with our limits, and wish to expand them. Let's use a concrete example. Say you've limited your scenes to a certain level of pain, and you decide that you want to try a little more. Maybe you just want to see what it's like to go farther, see if you can take it, see how you react to it. But you are not, voluntarily, able to accept it. When the pain reaches your limits, you safe out not because you want to, but because you have to! Afterward you always wish you had gone a little farther. Bottom line, you want your limits expanded.
Okay, now say you have a regular partner, whom you trust absolutely never to harm you or to violate your limits. Someone who is very experienced and knowledgeable. You decide that you wish to give that person permission to take you beyond your limits. You give him or her express permission to go beyond them. Understand that this is the only situation in which it is morally and ethically acceptable to go beyond limits!
What often happens is that once you've made that step across the limit, the other side is no longer unknown territory. Once it becomes known, it is usually a lot less frightening. And as we learn our limits, we often find that they are based more on fear than on actual physical abilities. As we experience what we fear, the fear diminishes.
Always keep in mind that emotional and mental limits are every bit as real and every bit as valid as any other! Violating limits that are fear-based is just as harmful to the well-being of the bottom as any other, and can be the most harmful of all! In some cases, it can leave emotional scars that may take years to heal, if indeed they ever do. Perhaps it's not quite as deep a wound as that caused by rape, but it can come pretty damn close.
And if your moral compass points South, perhaps this note of warning will sway you. Scene players look out for one another, and if you become known as unsafe to play with, you'll find yourself ostracized faster than you can say 'apadrawa'.
