The Edge of Extreme
Extreme play runs the gamut of BDSM. Keep in mind that what one person thinks is extreme is what someone else does on a daily basis. There is no one definition of extreme for everyone.
However, everyone's extreme has at least one thing in common: It frightens the played one, (often it frightens the Player as well). It is something that takes a piece of the bottom's terrors and toys with it. It is the fear of KNOWING that the bottom could die or be badly hurt. I'll give an example: blade/knife play.
In this example, the sharper the blade, the shinier the metal, the more fearsome the weapon. I know One Dom who uses live steel katanas. He shows how sharp they are by slicing through a melon tossed into the air.
I use a Turkish bora. It is an atomically sharp blade, as evidenced by the jeans I wear when I play with it. My jeans are full of holes where the blade has sweetly slid through the denim. I then blindfold the "victim" and proceed to scare the living shit out of him.
However, to Me, blade play, while inherently dangerous, is not edge play, (extreme play is often called edge play due to the pushing of boudaries. You're taking someone right to the edge of their fears). Both I and My slave have gotten somewhat used to it. Thus it is now just a delightful part of our play.
No matter what your edge play entails, there are certain safety concerns to take care of before you begin.
Experience
If this is your first time with such play, is it your Top's first time as well? Or vice-versa? If so, PLEASE have a standby available to offer guidance or help in an emergency if needed. Your standby should be very well versed in what you're trying, and if no one is available, DON'T. Be smart and hold off until you can find someone to help who knows what he's doing.
Learning Curve
Some play of this nature has a sharper learning curve than others. Don't underestimate what can go wrong. Be prepared for any contingency.
First Aid
The person on TOP should be at least certified in CPR if nothing else. There should be a first-aid kit within reach, (I find that not only is it in reach, but if the sub sees it before being blindfolded or before play begins, it either reassures them or scares the living shit out of them - both being enhancements to the play ), and you should be able to calmly handle whatever might occur. If you've had no first aid training, that should be something you do BEFORE you even think about playing. ALWAYS have a phone within reach and do not hesitate to call 911, (or the emergency number in your area), if need be. Remember: it's easier to explain why she's hurt than why she's dead.
God Complex
There's something to be said for the Top who knows that S/He isn't a god. They're called, "intelligent" or perhaps even, "real." I don't care who You are, You don't and WON'T know everything. Don't be afraid to have to learn something. Chances are, the sub will admire the fact that You're more concerned with safety than with ego.
Last thing...
When preparing to play with someone's terrors, leave Your ego at the door. At this point, no one is concerned with Your ego. The concern is with Your sub and with the careful, good preparation of a frightening scene. Edge-play is scary stuff, and You should know a bit of fear Yourself. Take control, be super cautious, listen to and watch your sub carefully, and have a good time.