If I understand you correctly, yes, the voltages (the commands) are seen by the built-in just as if the extra switch was not there (hence, the term, cascadable), BUT, the built-in can ONLY handle 4 STBs at once, now, the extra multiswitch CAN handle more than 4, but the built-in never has to know this, the x8 will translate any of the 8 tuners commands into a format that the built-in x4 can understand. The extra switch acts like a traffic guard of sorts (it takes 8 lanes of traffic, and puts it into 4 lanes, then it takes the reply from the 4 lanes, and inserts it onto the proper lane on the 8 lane road), it still passes all the signals to the built-in, but it prevents 8 seperate commands (if all 8 changed channels/voltages/sats at the same time) from confusing the built-in x4 by pretending to be 4 STBs, once the command is issued, the x4 sends the 'reply' back to the x8 (the x4, however thinks it's sending the reply to a STB, when it really isn't), now the x8 then reroutes the command to the proper tuner hooked to one of the 8 lines.
So basically, the built-ins becomes a 'dumb' relay, and the extra switch takes over as the 'smart' distributer to the STBs.