Hi,
First off, I stopped doing fuel additives in 1995, so anything I say is probably way out of date.
However, that said, the active ingredient in most injector cleaners will be a variant of the ashless dispersant technology that is so common in engine oils. The common feature of all of these molecules is that they have a long chain hydrocarbon, oil soluble end (eg Poly Iso-Butylene or Poly Ether) and a polar end (usually some polyamine such as DETA or TETA). The theory is that the polar end latches onto the crap in/on your fuel injector and the oleophilic end 'pulls' stuff into the diesel flowing through the injector.
There are so many chemistries that have been used for this job at one time or another (bis-succinimides, PEA, imidazolenes & stuff based on cresols/phenols) but they all conform to the same, two-ended molecule model. Generally these additives are bi-functional in that what works for diesel, also works for gasoline & what works for injectors, usually works for inlet valve cleaning.
So the big question is do they actually work?? I think the honest answer is they might but also might not! If you have a partially blocked injector, you have a chance of keeping it open. However if you have a properly blocked injector, where the tip is covered in hard soot & flow of diesel through one of the holes has stopped, then the additive won't make a fat load of difference. In such a scenario, you're probably better off physically pulling the injector and cleaning it mechanically.
Hope this helps,