CAGS Disconnect
CAGS, for Computer-Aided Gear Selection, is a misnomer. Given certain parameters such as gear selection (only happens in 1st), speed, throttle position, and engine temperature, CAGS causes 2nd and 3rd gear to be locked out, forcing a 1st-to-4th upshift. The theory is that by lowering RPMs, GM is cutting fuel consumption and, probably, noise pollution. I usually stayed in 1st longer to avoid it — so much for that theory.
I say it is a misnomer because it acts more like Computer-Thwarted Gear Selection. For example, I know I'm about to go around a tight corner that I'll want to accelerate through like you should. So I shift and then start turning. Unfortunately, the PCM decides it wants to be in 4th so you bog like crazy. But under normal acceleration the V-8 has plenty of torque, so it's fine. Starting in second or skipping 1st-to-3rd probably makes more sense.
Anyhow, it didn't take long for this game to get really old so I pulled the connection to the solenoid on the transmission and taped it off. Unfortunately for those with '96+ cars complying with OBD-II, you'll get a "Service Engine Soon" light if leave the circuit open like that. Fortunately you can fix OBD-II cars by substituting a sufficiently-rated resistor for the solenoid in the circuit.