In Gen4 applications, the throttle follower is quite literally that - airflow that 'follows the throttle'. When you're on the pedal, you have control of the throttle (most of the time). When you're off the pedal, it should return to idle control. However if you just snapped the throttle shut and went straight back to your idle airflow target, say after a pull at high RPM...the amount of engine braking would probably knock your head into the steering wheel, among other things. So the follower exists to allow a blend from the on-pedal airflow to idle airflow.
Might be useful to define what those airflows are, seeing as there is no 'base running airflow' table, as in Gen3. On pedal, Gen4 airflow is just what results from the throttle position as determined by driver demand. (Gen5 expands to include on-pedal torque control as well as off-pedal speed control, a true closed loop torque system...but Gen4 just has the speed control.) Off-pedal idle airflow at steady state is a function of torque, which is a function of engine friction, accessory frictions, alternator load, AC load, idle reserve...you get the point. All of these are summed up and input to the inverse of the airmass torque model where a cylinder air mass can be determined. From that and RPM, you have a steady state airflow.
Meanwhile when you're on-pedal, the values from the throttle follower table are also input to the air mass torque model and checked against the steady state airflow. When you let off the pedal, the difference between them is added to the learned integral airflow correction (depending on the LTIT cell values) and then added to the steady state airflow. It then ramps out depending on the follower increment/decrement tables.
Here's a screenshot from a 2011 truck E38, which has most of the CAN variables for this defined. You may not be able to see these on a lot of cars, as HPT doesn't have many of them defined. The leftmost arrow shows the tip-out of the throttle, at which point the throttle position ramps down to throttle follower airflow. The middle arrow shows the integral getting the TF airflow added in to begin the ramp out, and the last arrow shows the end of the ramp.
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Keep in mind...you could get airflow pulled out by the throttle follower integral rather than added in...just depends on your calibration and operating conditions.