how are the remote units told how much power to apply to the train, do they comunicate with the lead unit and if so how?
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Ironlake
DPUs communicate via radio. They can be run in unison with the lead units or controlled independently from the lead set of locomotives. The engineer can change between control configurations as needed.
I am sure some of our engineers or our fireman can provide additional detail.
Wait for Wyhog to reply. He's our resident expert on this.
http://en.wikipedia.org/wiki/Distributed_power
A locomotive that has been fitted with Locotrol DP equipment may be set up as either a Lead or Remote 'active' unit; the Lead unit being the controlling locomotive. Only one distributed power-equipped locomotive in any Lead or Remote consist (group) is active. Other locomotives MU-coupled to this 'active' unit operate conventionally as multiple units.
There are two basic modes for over-the-road distributed power operation. Locomotive control can be synchronous (MU), whereby control commands made by the engineer in the Lead unit are transmitted instantly via radio telemetry to—and are followed immediately by—all Remote units in the train, or independent whereby the engineer may set up and independently operate the Remote locomotives as a 'front' and a 'back' group (or with Locotrol III and subsequent versions; as 'Lead', 'Remote-forward', Remote-intermediate', 'Remote-rear', and 'Remote-trail' groups—this latter at the rear of the train). The front group always includes the Lead locomotive, and all Remote locomotives in the front group follow the commands made by the engineer using the Lead locomotive controls. Which Remote locomotives are in the front or back groups are selectable by the engineer in real time. One DP train cannot affect another DP train or another individual DP-equipped locomotive not in a train; and an individual DP-equipped locomotive not in a train cannot affect any DP train or other individual DP locomotive regardless of proximity.
Distributed power was originally able to be provided at only one intermediate location within a train. These forerunner systems (Locotrol 102-105 and Locotrol II) required a radio-relay car to be attached via standard multiple-unit jumper cabling to the remote locomotive(s) to provide the radio-control commands and facilitate feedback signals. Later, Locotrol II evolved into the 'Universal' system in which the radio-control equipment was installed on the locomotives themselves, rendering the relay car (variously referred-to as an 'RCU' for remote control unit or 'LRC' for locomotive remote control) redundant.
Locotrol III was the next development—being compatible with both the Knorr-Bremse / New York Air Brake CCB and Wabtec's EPIC electronic locomotive brake equipment, and permitting multiple Remote unit locations as described above. The latest incarnation of this equipment is Locotrol Electronic Brake (LEB), which integrates the GE Locotrol technology with K-B/NYAB's CCBII brake .
Already written up in Trains Magazine, quite some years ago.
Originally Posted by Wyhog:
...
3. While hostling at Alliance, Nebraska i used to move the remote consists (3 or 4 units) around the roundhouse service tracks separately via remote control from a master consist. That way I didn't have to get out in the sub-zero cold to walk from one consist to the other. It worked pretty good for a week or so until one night the roundhouse foreman climbed up on the moving slaves to inform me of some work changes and he found no one on the moving locos. He dynamited the slaves and I walked over to see what happened to them. It was then that I learned I was NOT to EVER do that again. 
That's a helluva story! We're glad it worked out ok in the end, but would have loved to see the look on his face when he saw the ghost at the controls!
