The unit ties into the existing OEM loom using a variety of signals, such as injector pulse, RPM, speed etc... it must do some funky calculations based on this signals. In each gear it has an oscillating numeric reading, but each gear's output is unique, so that's how it knows.
Stephen has installed these on 930's before, so he'd know how to hook it up in your application.
'89 911 Turbo Cab
Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition