does Comcast use traps?

do they work by blocking signal?

if so, how can I get digital stations thru my QAM?

do they physically block premium channels, scramble them, or both?

It's going to depend on where you are located. generally speaking, they do not trap out the Digital channels, since they are in small 6-MHz "bouquets" scattered throughout the spectrum (many of the channels that appear "vacant" on an analog set, are really digital).

With Digital Cable, every STB is addressable, and most premium channels are scrambled. You need their box to access their scrambled channels. Many Comcast systems pass local HD (and, often all local stations in digital mode), but you should check out your local area thread for details. If the answer isn't already there, ask the others on that thread.

Many channels, are encrypted, and can only be decrypted via a rented STB (individually addressable) or via a rented CableCARD (also individually addressable). Local broadcast analog channels are available in-the-clear everywhere. Local broadcast digital channels are available in-the-clear here in Burlington, but not everywhere Comcast serves. ADS versions of local broadcast analog channels are very infrequently available in-the-clear -- not enough to rely on for use. Cable analog, non-premium channels are available in-the-clear here in Burlington; I'm not sure about elsewhere. Are there any places left where they still provide analog premium channels? Not sure. Cable digital channels (premium and otherwise) are all encrypted, and I believe that's true everywhere.