What is a power jack and why would you want one? A good question. Simply put, the power jack replaces both the switch and the receiver battery charge jack in your airplane. No more having to deal with connecting and disconnecting wires sticking out of your plane in order to charge the internal battery. No more replacing the super cheap switches most RC radio sets provide.
The power jack is very unobtrusive (see Figure #1below). The power jack has a plug in place when you want to leave the power disconnected, or you want to charge the internal battery of your plane. When you remove the plug, you battery supplies power to your receiver. Very simple.
Some care is needed. This is a popular thing modelers do, but they often do it with substandard jacks. Sometimes even using stereo jacks. Stereo Jacks are not designed to deliver DC power ! If you are listening to your stereo and the jack fails, all you get is some crackle. If it fails in your plane, the result can be a disaster.
You must use high quality DC coaxial jacks designed for this purpose. I use and recommend the Switchcraft 712A power jack used with the Switchcraft 760 power plug. Both are available at many electronic supply houses, or online at http://www.mouser.com/
Currently I am using Hitec radios only, this works fine for those. Any others brands, try at your own risk. As with anything else, this requires care, and I cannot personally guarantee results. It does work consistently for me; I have them in every new plane I build.