Ten or so years ago a company came out with a revolutionary seat/post designed called Pivotal. This has been the standard for a long time now. It seems like BMX wasn't satisfied, and decided to try going another route. But Kink was not on board just to save a buck. So Kink came out with the Stealth seat system that still utilizes the Pivotal system.
First, let's talk about the Pivotal system. Originally a pocket on the top of the seat has a 6mm hex bolt that goes directly through the seat controls the entire seat/post. The post has a hole in the center of it that accepts that bolt, and the teeth like parts on the seat, and seatpost allow the seat/post to be adjusted, on a pivot.
The Stealth seat/post is uses the Pivotal system, but uses a unique mounting system to secure the seat to the post. Rather than going from the top of the seat through the "coin pocket," they've broached the Pivotal bolt from the bottom to accept an easy to find 5mm allen wrench. The threads are reverse on the bolt, so tighten to the left when you're looking at the underside of the seat/post. To achieve the ability to use that broached bolt, Kink shifted the lip of the Pivotal post and put the bolt on a more natural angle.
Many people think this angle reduces the angles, but I can tell you from personal experience that it does not. The clean look, with the same adjustability as a typical Pivotal seat/post, you can't go wrong with this. Or you could go on and use that other system that doesn't allow you to keep the "free" in freestyle with your BMX parts.
Posts come in many sizes, from 75mm, 180mm, and 330mm for you old school guys. The posts come in Matte Black, Red, and Raw. This seat comes in both Medium(pictured), and Fat. Weighing in at an average 10oz, with a bad ass Microfiber cover with an embossed Kink K-Star Graphic in black only. You should be able to get this for around $31.95 at better bike shops and mail orders.
Tools needed: 5mm allen wrench, possibly a 5 or 6mm for your seat post bolt. Remember to GREASE your Pivotal bolt, and your seatpost shaft so your seatpost does not become ceased in your frame.
For more info on this, and many other Kink parts, check out kinkbmx.com