The STB can only decode one channel at a time. When it is switched on, this is what it outputs to the RF output. On SOME boxes it is possible to also receive the analogue channels that are also sent down the cable. Usually, channels 1-5 plus a few others are available unencrypted.
Check your box against this table:
http://homepage.ntlworld.com/altisuk/temp/stb.xls
The feature you need for this is called 'internal bypass'.