They're replacement fuel pressure regulators set to a higher pressure than standard. The amount of fuel injected is a function of injector pulse width and fuel pressure so they, in theory, provide more fuel than the ECU planned and maybe a bit more power at the expense of economy.
However, as well as having a pre planned map for fueling, the ECU also learns how well its doing by getting feedback from the exhaust gasses via the lambda sensor. If it finds its regularly over fuelling, then it adjusts the map offset to compensate.
So in short unless you've got a modified engine requiring more fuel than the injectors can normally supply, they're a waste of time imo.
Complete waste of money unless you have a tuned engine, and if you do have a highly tuned engine then they are still a waste of money because you'd need to change the ECU (and possibly the injectors), in which case you'd alter the fuel mapping anyway .