Strobist question

[iND]

Question

When I am using my 580exII on camera and am using the built in IR to trigger two other 580exII off camera
AND
I am using ETTL on all units with the on camera as the master
one off camera speedlight set to A and one set to B
I can balance A B on the camera menu no problem

Ok so here is the question
if the metering is done by a preflash on the master unit
first why does the preflash not set off the slaves
second will the preflash set off a monolight (too soon)
third if the master takes a reading from the preflash determines how to distribute the flash to the slaves by the ratio set in the menu
Does it know how far my slaves are from the subject?(flash fall off is a square of the flash to subject distance)