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 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).