When I first heard of play piercings I remember thinking of something I used to do as a child. I used to take straight pins and slip them under just the first layer of skin on my fingers. It didn't hurt, and it only bled if I went too deep. I would do it because it felt weird, and it freaked out my mother. Play piercings are similar to this. 

Many insert the needles or such just beneath the first layer of skin. Other, go further, inserting the needles into the muscle beneath the skin. It seems to depend on the sensation the people are going for and the reason for the piercing. 

If the piercing is to be used to suspend the submissive, then the needles must go into the muscle in order to hold the weight of the submissive. This is often called "Fish hooking" because fish hooks are used. If done properly, the submissive can be suspended off the floor by the needles in their skin. This is dangerous and should not be tried without some serious thought, discussion and TRAINING. 

Some use play piercings to attach something to the submissive. Like a sign, or a ribbon in a decorative pattern. For example, using needles in two rows along the back, can allow a ribbon to be weaved around them in a pattern similar to the closings on old corset styles. I've heard of submissives having a note, or sign, pinned to their skin as punishment, advertisement, or any other number of things. Play piercings can be similar to accupuncture. Where the neeldes are just embedded into the skin, in certain areas.  

Engaging in such play requires a tremendous amount of trust from the submissive to the dominant. Such a scene can be a way for the submissive to give an overt show of their trust and submission for the dominant.  

Play piercings are considered by many to be a form of edge play. They should not be done lightly and definitely not without some training in proper techniques. It is a very dangerous way to play. Certain safety procedures must be followed when engaging in such play. It is best to research such play, and witness such personally before attempting it on your own. It is a must that the needles be sterile, and sharp. Dull needles wil tear the skin, instead of piercing it, which can result in scarring, excess bleeding, extended healing time, and infection.  

Needles should never be used on more than one person. If such play is done as a one shot deal in a play partner arrangement, the needles should be disposed of rather than saved to be used on another submissive. It is very difficult to remove all blood from a surface, so it is better to be safe than sorry when it comes to any play that includes blood. There are areas of the body where needle play should not be attempted so a good working knowledge of basic human anatomy is a must. Inserting a needle into a nerve bundle, could have permanent and disasterous results. Some nerve bundles are found in the wrists, ankles, knees, under arms, shoulder blades, and along the spine. Having a good knowledge of human anatomay can prevent accidentally harming an underlying nerve bundle with needle play. The needles themselves should be thin, such as accupuncture needles as these will pierce the skin easier and not cause tearing of the tissues around the entry wound. 

Research, learn and practice before ever trying this on another person and you will find the experience to be more satisfying for everyone involved!