Positive traps are used in combination with encoder systems to restrict the delivery of certain frequencies over the cable. They don't come into play until a user chooses to have an encoded system delivered. For instance, if you choose to buy a premium channel through your cable box, your positive trap will filter out the encoding frequency, releasing the decoded frequency and the new channel to your cable box.
Negative traps block certain frequencies from traveling through the cable. They're more cost-effective than positive traps but also more prone to tampering. In order for the frequency to be unblocked, the negative trap must be removed.
Certain positive and negative traps just restrict audio or sound. Other traps are combination positive and negative traps, because negative traps can sometimes cause interference to nearby signals that are allowed to pass, creating fuzzy signals from frequencies traveling too close together over the cable.
CATV traps are vulnerable to humidity and exposure to weather. The best traps are hermetically sealed -- air-tight -- and protected from the elements. Accessible cable boxes can have their traps altered and hacked, which damages the overall quality of the cable and cable service.