I was thinking about buying a cable box descrambler. How do they work, an what's the best one to buy

Many cable companies have discontinued analog services or are in the process of phasing analog services out. You probably shouldn't expect an analog converter or analog "cable ready" TV to be able to receive cable signals for more than a few years. On some systems they are already useless without the aid of a digital converter.

Many cable companies distort premium channels (like the analog versions of HBO and Cinemax) in a manner that requires an addressable converter for a remotely viewable picture. The converters default to not descrambling those channels until the converter receives a special code to allow reception of that channel. Each individual converter has a unique "address" code for those channel activations. Many cable companies will only activate converters that individual company rents to their customers. "Missing" converters that someone failed to return upon canceling their cable subscription (local or from another region) might be regarded as stolen property. The converters might even default to the non-receiving state after a short time without being connected to a live electrical outlet.

At their convenience some cable companies dispatch technicians to install filters in the drops to the customers residence to block many out of town channels from even reaching the building. Those filters are sometimes installed high on a utility pole, an apartment building's utility closet or in a padlocked distribution pedestal (where there are underground electrical and communications utilities).

While the technology is different digital converters and cable cards are managed in a similar manner. A digital converter or cable card that wasn't obtained through the local cable company's rental program might be completely useless.