How do inline or piggyback programmers work?

I have an 03 G35, just purchased a jet vforce plus piggyback module. It only uses 6 leads total. One is for 12v, one for ground, one for O2 sensor signal, one for tps signal, and two for maf sensor. (MAF harness wire is cut and the two wires from the module simply splice in line and reconnect the loop.) with this set up, what parameters could the module change without going directly to the ECU? Does changing the maf readout really affect spark timing and fuel maps enough to get the hp gains that jet boasts about? (avg 12 hp gain)...