The three pulses from the mater are 1) pre-flash command, which tells which group should do the pre-flash, 2) pre-flash trigger low, and 3) pre-flash trigger high. The last two are just simple double-pulses.
The remote unit emits two pre-flashes immediately after seeing the pre-flash trigger signal. The second one should be stronger than the first one. The 2nd one is requested if the camera sees a weak return from the first pre-flash. It will skip the 2nd pre-flash if the first one is good. At least this is the case with a SB-800 as a master. With the built-in flash of D200, it always asks for two pre-flashes no matter what.
Remote unit does not communicate anything back. I emits only the pre-flashes and the final flash.
All remote flashes in the same group will fire with the same flash output amount, at the same time.