[Shankenstein]

*slaps forehead* piggyback! That's an important detail I somehow missed. Totally different ballgame then.

To help people understand:
Redline (and fuel cut) is an event that's usually attributed to pulse frequency on either the crankshaft or camshaft trigger wheels. Usually, the number of clock ticks between TRIGGER_UP events (or missed teeth) will be used to estimate engine speed. The estimate is heavily filtered before the ECU uses it.

The factory sets a threshold value for engine speed and performs an event such as fuel cut (injector return wire isn't grounded), spark grounding, etc to limit additional power.

Many piggyback systems monitor the injector signal for the voltage rise/fall associated with start of injection. They calculate the new pulsewidth and lengthen the ECU's command. It's an event-based trigger method that compares current voltage to a threshold (with some complicated calculations in there).

If there's fuel cut, there's no trigger... and no trigger means, no fuel (ECU or piggyback values). There are some ways around it, such as tricking the ECU to continue fueling, or generating your own waveform. Most solutions are pretty hairy, and new cars come with alot of supplemental systems that have nanny-control (such as the FR-S automatic's "stupidproof" manual mode). Not simple, and not guaranteed effective.