Once upon a time the crane stock was an exotic tactical upgrade over the standard M4A1 stock as part of the SOPMOD system but it has since grown to become a fairly standard fair basic upgrade component for even the most mildly modified M4s. The Crane stock attaches over the stock tube of an M4 / M16 weapon just like the original stock and just like the original it is telescopically adjustable to 6 positions. This stock is different in that it incorporates additional interior space; just enough to accommodate the crane stock style tri-stick type of battery (9.6v or 10.8v) allowing you to wire the battery to the rear.
The stock itself is a very durable, fairly lightweight (though heavier then a standard M4 polymer adjustable stock) piece which hold up well to long term wear and tear. The butt plate is both rubberized and textured to improve grip, hold and comfort to a degree.
This is particularly useful in M4s with rail systems as they lack the space to store the battery in the front and externally wiring a battery box is not the neatest or prettiest solution. Also, the rear wired battery helps to pull a bit more weight to the rear of the weapon which goes some measure in balancing the weapon a little better (although not much if you are using a metal rail system anyway).