On diesels, all tunes inject more fuel to make more power. They do this mostly by increasing fuel pressures so that more fuel is injected during an injection event (all of the tunes use this technique). There is also some play in the duration of injection events.... But injection event timing tuning is fairly limited (diesel burns slow and the size of the injection window is small). The one thing DDE tunes can do that piggy backs cannot is also increase the amount of air (through increasing turbo duty cycle) so that all the extra fuel is burned properly (less smoke) and more air keeps EGT temps down.

Anyway the one piggy back tune many, many folks have used with good results is the Burger Tuner, it works by increasing fuel rail pressures. The problem with this approach is the DDE detects too much change in fuel rail pressure and throws a code, or can over time adapt around it (it has O2 sensors and a MAF and can detect too much fuel). So its effectiveness is limited, but its easy to add, easy to remove and cheap. It does work, and I think it's a good starter options, lots of folks have run them for years without issue (I did). But it's not gonna add a ton of power, about the same as a stage 1-2 tune would with stock components.

All tunes run the car outside its original specifications and eliminate engine features that are designed to prevent this out of spec running. It's what a tune does, run the car out of spec.

You can run a piggy back and a tune on the same car, some folks did this until the tunes caught up to compensate for fuel rail pressure drops at high RPMS. There are also upgraded fuel pumps available for the diesels when you need more pressure and volume... BMW R90 HPFP Pump for 335D and X5 35D