Transmissions Physics
In order to appreciate what technological marvels these modern semi-automatic transmissions are, you have to look at what a computer controlled transmission and engine management system has to figure out every time a gear change is performed. Because the transmission is driven at the same angular velocity as the wheels, while the engine is doing what your foot tells it to do, the clutch can only be engaged when the engine and input shaft are spinning at the same speed.
In a normal manual transmission, the engine speed naturally drops when your foot comes off the gas, right before the clutch is disengaged. This works perfectly for an up-shift since the new higher gear will also require a lower input shaft speed. Both the engine and input shaft are decelerating, or experiencing negative torque, as the clutch clamps down on the flywheel. Any difference in speed can be taken up by the slop of the clutch and how fast it is re-engaged. Downshifts work the same way, but in reverse. Since any self proclaimed car guy would already have mastered the technique of rev-matched downshifts, the heel-toe throttle blip speeds up the engine to match the now-higher input shaft speed of the lower gear and magically everything comes back together because both the engine and input shaft are speeding up and experiencing positive torque.
What most people forget when driving and critiquing one of these new fangled semi-auto gearboxes is that they drive it like an automatic. Nobody ever lifts while shifting, yet they expect shifts to be fast and as seamless as an automatic, while offering the immediate feedback of a dry clutch. Lifting makes all the difference in the world. While designers expect these transmissions to be driven with the throttle mashed during up-shifts, physics is against a smooth shift. Without lifting during an up-shift, the engine continues to rev and generates positive torque while the decelerating input shaft on the transmission is generating negative torque. Regardless of how quickly the gear change occurs, the re-engagement of the engine and the driveline, through the clutch, will cause shock and excessive wear, just like a power shift. Torque converters can tolerate this, due to the hydraulic slip built in, but even up-shifts in an automatic are far smoother when the throttle is slightly lifted to decrease the torque difference. Race transmissions address this by interrupting the spark signal during the shift to temporarily reduce positive torque coming from the engine, but this has huge emissions consequences, which can't be used on a production road-going vehicle.
More modern vehicles address the torque issue by using electronic throttles that are programmed to control engine speeds during a gear change and control the torque conditions. The e-throttle is critical for any modern electronically integrated semi-automatic transmission. During a full throttle up-shift, the throttle closes slightly to reduce torque and match the engine and input shaft conditions. E-throttles are also how throttle-matched downshifts are achieved by quickly opening the throttle to increase revs, like a throttle blip.
But full throttle up-shifts and downshifts on the track are actually relatively easy to control since engineers can program e-throttles for the exact amount of throttle blipping needed for downshifts at the limit, as well as throttle closing during up-shifts. All an engineer needs to know is the specific gear spacing (which translates to the RPM drop or gain between shifts), engine torque output, and the amount of throttle opening or closing, as the clutch is disengaged, required to bring the engine and input shaft to the same angular velocity. With these variables known, how fast and how hard a clutch is disengaged and re-engaged is a matter of software calibration preference. This is why the cars that we've tested can often better predict gear changes and can even shift faster in full drive mode when on track, rather than when manually shifted.
Transient driving conditions, in the city or in traffic, are far harder to manage, as predicting a driver's intent under all conditions is nearly impossible. Even with the help of the steering angle, throttle position, yaw, and wheel speed sensors that these semi-automatic cars are all equipped with, a vehicle cannot be programmed to predict a change in lights or a sudden vehicle avoidance maneuver until things begin to happen.
Say you're coming to a stop for a left turn in second and slowing down. Since there are no steering inputs, the engine and transmission computers anticipate a complete stop and are ready to downshift into first when the vehicle stops. Then the left turn light suddenly turns green and the throttle is floored to whip through the turn. But the car has already slowed to the point where second gear is well out of its power band. The computers can't do anything other than bog through the gear to get through the turn.