Check out AKMike's good review. For other relative newbies like me, this tool is only needed if the rear of the brass case gets completely ripped off, and you end up with a peek-a-boo see-through spent case stuck in the chamber. To extract the stubborn case remnant, insert the skinny end of this tool though the stuck cartridge. The spring loaded 'tines' on the front (kind of like a drill chuck) act as a barb after you push them through the case neck. Then close the bolt, the extractor should catch on the rim of the tool, just as for a regular cartridge. Then just pull the bolt back, hopefully with the tool and 'impaled' case remnant.

Keepshooting offers this at a good price, compare with some of the big firearms parts vendors.

Though I haven't experienced a de-capped case, I've read this is more likely to occur with hot loads or cases reloaded multiple times (brass becomes extruded or brittle with use, especially with improper headspacing).

With the growing popularity of m4s, decapped cases are becoming more common. With the shorter gas tubes and gas ports being drilled closer to the chamber, you get a much quicker and overly powerful extraction. It starts the extraction while the case is still expanded against the chamber walls which means it has much higher resistance. This means that if a case isn't up to par or the round is loaded a little hot, you can experience breakage. Assuming you didn't just ram the next round in deep enough to fire the weapon resulting in explosion, you can clear the broken case quickly and easily with this handy little deal.

I have learned that your shooting time will be cut short if a case head tears off and you either don't own one of these or you left it at home. Buy at least one per cal. Put it in your range bag or rifle case or what ever you have to. You could be toast with out it in a bad situation.