I'm no expert but my understanding is the "old consist" style is where the controller sends out individual messages to each loco in the consist. So if you had locos running on address 3 and 5 say, then when you set speed step to 1 for the consist the controller would send out speed 1 to loco 3 and then speed 1 to loco 5. With an advanced consist, there is a CV that is used to make the loco listen on another address, say 127, so all locos in the advanced consist listen for messages addressed to 127. Then the controller sends out one message for speed step 1 to just loco 127 and then both loco 3 and 5 would respond at the same time. So basically no delay in the message for the speed step, which helps if locos are double headed, for example.