it works quite simply.
the mce machines knows what tv program is on and at what time, due to the guide data that is downloaded
it ALSO knows what channel the stb must be on for that program to be shown (this is all collected automatically via the epg download)
and once more, it knows what ir codes need to be sent to make you set top box change channels (it either learns your existing rmeote or you tell it what stb you have from a given list)
then, you plug a little ir blaster supplied with the mce machine into the back of the mce machine, and place the iremitter by the ir window of your stb (normally in the front)
now, when a recording is due to take place, mce will send the appropriate ir codes through the iremitter and your stb will change channel accordingly. winderful
the only caveat is the pace series 4000 (4000, 4001 etc) set top boxes supplied via ntl and telewest cabled areas. They only "speak" irda, whereas ALL mce machines output CIR (consumer infrared), hence the irblaster cannot make it change channels
www.redremote.co.uk sell a cir-->irda translator specifically for this purpose, expect to pay about £35 for the device.