One of the more popular items used for impact play is a crop, which is a flexible shaft, wrapped in leather, with a handle at one end and a loop at the other. For the purposes of this page, I'll consider a bat, which is the same basic idea only typically a little shorter than a crop, as being essentially the same thing.
Crops are used by equestrians and BDSM fans alike. They provide a light to moderately heavy stinging sensation, and can, if that's your thing, be used to create very distinctive bruises on your partner's backside as well.
BASICS OF CROPPING, AND WHAT IT FEELS LIKE
Crops are a lot of fun as sex toys. They produce a sharp, stinging sensation, more localized and potentially more intense than a hand or a paddle, and can be used either with great delicacy (in which case they don't hurt at all) or with more force (which is quite stingy and can leave nice bruises as a reminder of the fun you had the next day). If you like spanking but have never experimented with a crop, I encourage you to try; they're inexpensive toys that bring spanking to the next level.
There are several different types of crops and bats that can be used for BDSM play. Generally speaking, a good crop should have nicely finished wrapping around the shaft and a finished loop on the end without rough or frayed edges. Some crops and bats have sewn ends, which should be double-stitched and free of loose, frayed, or rough edges. I've seen poorly-finished crops available as novelties in some sex toy stores, which I don't recommend.
The primary difference between a crop and a bat is the length. A bat is shorter than a crop, but is otherwise similar. All other things being equal, bats tend to be somewhat less intense than crops when used with the same force. Some people use the term "crop" to refer to both bats and crops; in the "using a crop" section, I'll refer to both bats and crops generically as "crop."
Bats and crops are also available with a variety of different "keepers" on the end, which may be a simple loop of leather, a flat stiff piece of leather, or even a shape such as the hand shown above. The size of the end makes a difference to the way the crop feels as well; generally speaking, all other things being equal, a crop with a narrow end (like the bottom example shown below) will produce a sharper sting than a crop with a wider end when used with the same force.
Different crop designs also feature different materials and designs on the shaft and handle, though these differences are cosmetic and don't change the "feel" of the crop.
Using a crop
If you're unfamiliar with a crop, the butt is probably the best and safest place to start. A couple of things to keep in mind: When you use a crop, you want the end of the crop to strike flat against your partner's skin. You don't need very much force at all to create a strong sensation, so start off light until you and your partner are familiar with the crop and your partner's limits. Striking the same spot again and again will very quickly amplify the sensation.
I've met only a small number of people who enjoy a firm cropping right off the bat; most people require a warmup before you can use the crop firmly. An open-handed spanking often makes a fun and sexy warmup. You can also use the crop itself, tapping it very lightly and quickly on your partner's backside before you start gradually increasing the firmness of the strokes.
Because the crop is flexible, you can easily find a rhythm that allows you to strike in steady, rapid strokes. Have your partner lie down on his or her stomach, or bend over a bench or the end of the bed. (Keep in mind that if your partner is bent over, this stretches the muscles in the butt and legs, making the crop feel more intense.) Run the end of the crop lightly down your partner's back and over your partner's buttocks. Take your time; there's no need to rush, and the teasing will increase the anticipation. Stroke your partner's skin with the end of the crop, then start a light and rapid tap-tap-tap of the crop on your partner's buttocks. Move the crop as you do this, paying attention to both buttocks and bringing the end of the crop slowly up and down the curve of the butt as you tap.
When your partner is squirming under the crop and the endorphins are going, it's time to kick things up a bit.
Begin alternating steady, rapid tapping with harder whacks: tap tap tap WHACK tap tap tap tap tap WHACK tap tap. Alternate between left and right buttocks, and keep a steady rhythm so that your partner knows what to expect. Playing with the anticipation is half the fun.
Something I like to do once my partner is warmed up is to press the end of the crop against one spot and keep it there for a while--several seconds to a minute or more--then, without warning, give a lightning-quick thwack right on that spot. Psychologists say that the anticipation of pain is sometimes as intense as, or more intense than, the actual pain itself, and as with many things about the human psychology, that's something you can put to kinky use in the bedroom.
Another variant on this same idea is to stroke or lightly touch your partner with the end of the crop while you describe precisely what you're about to do: "In a few seconds, I'm going to give you three solid smacks, right here."
Beyond the Butt
There are many parts of your partner's body which offer fun and interesting targets for a crop other than the butt. You can use a crop on your partner's inner thighs, for example, which many people find to be an amazing sensation. If you use care, you can also crop the breasts, though you should start light and work up especially slowly.
I have met women who can orgasm just from having their thighs cropped. Cropping here is particularly effective if your partner is bound in a seated position (to a chair, for example) or has a spreader bar holding his or her legs apart. Begin at the center part of the inner thigh, tapping with the end of the crop with light to moderate firmness. Strike in a steady rhythm, gradually increasing the energy of the strikes as the endorphins kick in. Alternate between the thighs, working your way up higher and higher as you go.
If your partner is bent over a bed or a bench or something similar, you can crop his or her inner thighs from behind, and alternate cropping here and cropping the butt. With a long, flexible crop, you can place the keeper between the thighs and move it back and forth rapidly; you will find a speed at which the crop's flexibility will cause it to easily and rapidly slap both inner thighs. (Try it, you'll see what I mean.) This is more difficult to do with bats, as they're shorter and less springy.
When cropping breasts, try moving the crop in circles around the nipple, or alternate light, rapid taps above and below the nipple, until your partner gets nicely warmed up. You can strike directly over the nipple, but take care and experiment slowly at first; some people enjoy this, some people don't.
Cropping the breasts is a great way to get the endorphins flowing. You can switch back and forth between using the crop and using other techniques to stimulate the breasts, such as a Wartenberg wheel. You can also use the crop to get the blood flowing through the breasts and nipples prior to clamping them, though clamps have a way of making nipples so sensitive I generally don't crop the breasts after using them.
Because cropping is psychologically powerful and as a way of getting the endorphins flowing, it's a great addition to role-playing scenes, such as teacher/student or even interrogation scenes. Experiment, find out what it does for you, and have fun!
What you'll need
Good, high-quality crops and bats can be found easily at just about any tack or equestrian shop. You can sometimes find them at sex toy stores as well, though I recommend being cautious with these. Inspect them before you buy; cheap novelty crops are often poorly-made, and may not last long. Look for rough edges on the end, roughness in the leather wrapping around the shaft, and loose or poorly-fitting handles; these are signs of cheap "novelty" crops that I advise staying away from. They're better used as props or part of a costumes than as real toys.
If you prefer to look online, here are some good bets.