MIRROR: Download from MEGA
Most forms of pain play that cause significant pain will leave some sort of mark, although moderate cropping and paddling and even flogging will usually only leave marks that last for a few hours. So if you stop before you start seeing welts, there's a good chance that you won't leave serious marks. But, some marks may only show up after play finishes, and some subs bruise more easily than others. So, in essence, any form of impact play that goes beyond light impact carries some risk of bruising, welting, and other marks. Take time to learn how your sub's skin reacts to impact and you'll probably learn what will mark and what won't.
Tit torture, such as pinching and clamping, will usually not leave bruises, but after a more aggressive session, the nipple will often crust over in the following day or so. Vaseline and other ointments seems to reduce crusting a little.
Genital torture (like clamping) may not leave too many marks, although personally I don't have any experience with lady parts.
In general, clothespins and clamps are one easily way to cause pain without too much risk of marks. Clothespins can be put on all sorts of painful spots--earlobes, lips, tongue, nips, the webbing between fingers and toes, genitals, and any skin you can pinch elsewhere.
Hair-pulling is another option. If you want to be really nasty, tie her up and then pull out body or pubic hair with a tweezers.
Electro is certainly good for pain, and I don't think (from what I've read, only a little personal experience) that it's likely to leave marks. But it requires expensive equipment, and you need to read up on what's safe and what's not (or find someone to teach you). A similar option is hot wax; most of what I said about electro is roughly true for wax (except that it doesn't require pricey equipment). From what I've read, wax only leaves redness that will fade in an hour or two. And wax play leaves the skin sensitive and ready for other forms of torture.
Fileboom Premium Account
Keep2share Premium PRO Account
