There is only two ways for a CR chip to work.
1) Rail Pressure adjustment (Roo/DTE/Tunit/DP all the same chip)
2) Injector Pulse Width Extender (Steinbauer, DTE)
#1. Is normally lowering the return signal coming back to the ecu.
Downfall is if you go to far the ecu will see this low signal, compare it with what its demanding and if it is out of spec put it into limp mode because it thinks there is a fuel leak.
#2. Wont put the car into limp mode as the ecu has no idea more injector opening time is happening. Rail pressure will drop slightly when the injectors are open for longer, resulting in lower atomisation.
Obviously the best bet is a mix of both. Little bit of pressure to bring atomisation level up.
And injector extension so the ecu wont register a fault.
The standard ecu controls its fueling based off water temp as it is much more stable to use than just EGT. So either way you go, once things start heating up, the ecu is pulling fuel out anway.
This is the reason chips make a good gain through one power run on the dyno, BUT! as the temp will rise quicker the power will be pulled away quicker also.
Normal drop in power is 5% after 60seconds at full throttle standard
15%+ after 60seconds full throttle with more fueling if no more air is added to sustain the higher fuel load.
Also, pre CR chips work in a few different ways to.
#1 Adjust spill valve directly via extending the falling edge of the signal (most chips).
#2 Adjust the ecu signal going to the spill valve control unit (unichip).
#3 Air flow meter adjustment on some vehicles that is done mainly through the canbus. Unichip is the only chip I know of that takes this approach on some vehicles and combines it with ecu output signal manipulation to control the spill valve.
One other thing to remember is fuel pressure/spill valve control is just one aspect of running a diesel engine.
The ecu determines how much fuel and when to inject it based off alot of information.
RPM/Throttle Position/Coolant Temp/Air Temp/Altitude/Boost/Gear/4WD selector position.
Most aftermarket chips use one input. Either rail pressure or current spill valve voltage as their only input.