i believe the dongle is constantly sending dmx data out (that's why you are NOT to turn on the dmx addin inside vixen) then just updates the dmx stream, as the sequence is running.
if no data was sent out on the dmx. sometimes, the wires can pickup stuff and create random flickering, so by sending constant dmx with each channel value to "0" it avoids teh flickering. (I could be wrong on the specifics, but i think i got the general idea down)
even though the values sent are 0's, the channel numbers are sent too, so the whole steam is sent wtih 1's and 0's at a fast rate. and because the human eye, can't see that fast (persistance of vision), and i don't think the led's can respond that fast. you get what looks like an on, or slightly dimmed led.
i hope that helps, and i tried to stay in layman's terms.