Knives are multitaskers. Knives are surprisingly versatile tools in the hands of a kinkster. The size, shape, and sharpness of a blade can make it suitable to scrape wax off of skin, hold to someone's throat with relative safety, or in the case of scalpels, carve intricate semi-permanent patterns into the first few layers of skin. It's beyond the meager scope of my little Kink Corner to describe which knife types and sharpnesses go with which activities, so be sure to research well—preferably with a mentor—before setting blade to flesh in any setting.
Knives can be mean. There's a reason knives have that reputation! More than any other commonly-used kink item, knives are apt to elicit visceral reactions, both from the knife wielder and the knife “victim.” Knives are very personal weapons due to the close proximity necessary to use them, and while that's perfect for putting a dangerous little spin on an erotic moment, the lack of distance can sometimes throw both parties for a loop. Be prepared (and that includes making sure your partner's cool with knife play before you whip one out, either to use on them or have them use on you).
Temperature play is hot (and cold). Knives don't need to be used as a threat; when heated or chilled, the blade of a knife can elicit entirely different sensations. The back (unsharpened) edge of a cold knife can feel very much like the business edge when there's a blindfold and a little mindfuckery in play. Or try it with the blindfold off as a way to ease into knife play when the “victim” needs a slow introduction.
Swordplay is in vogue. For all you size queens out there, swords are gaining popularity as a blade scene instrument. Training is essential, but the flexibility of the sword as a plaything is worth it to many people. Unlike smaller blades, it's possible to use the flat of a sword as a percussive tool, although it should be obvious to you, dear savvy reader, that hands-on training before attempting such a thing is an absolute must, lest you turn your partner's incredibly sweet ass into hamburger.
Sometimes, a knife is all you need to make a point. The ingrained psychological reaction most of us have to knives can be a great tool in and of itself. I bought metal handcuffs before I knew better than to use them in a scene (now that's a whole other column), and I still keep them in my toybag so I can throw them on a table with the rest of my implements just to get a rise out of my bottom. A knife can be used in much the same way; sometimes seeing one is all it takes!