The P version is a potentiometer, it has a stop and start points like an old DC power pack. Smooth in between a definitive start and stop.
The standard version has no start or stop point. The knob spins continuously with small micro indents.
A potentiometer or “P” type cab is an ABSOLUTE speed control. Its position indicator gives you absolute feedback on the speed position relative to the stop and start points. It is the same type of control as a value control one would find on an electric guitar or a older stereo system. So if you set the knob halfway between stop and full, you will have half speed.
In terms of running a single train, the P throttle is easy to understand and use since it like what a DC throttle would have for a speed control. Where it falls apart is when you want to change trains without stopping the current train you are running.
If you running train A at half speed, and then select train B which is stopped, train B will suddenly take off and speed up to half speed as fast as the decoder allows. Likewise if you stopped a train you have selected, set the throttle down and unplug it but accidentally knock the throttle knob to a non stop position while it was unplugged, then plugging in the throttle will result in the train suddenly moving at the speed set by the knob without you doing a thing. One of the known mysteries that people have when they turn on the layout after it was shutdown and are surprised to see a train suddenly moving when you turn the power back on can be directly traced to a “p” throttle knob not in the stop position!
Bottom line is controlling two moving trains at the same time becomes very difficult with a “p” throttle and you must be careful using the throttle or you will falsely think you have a runaway train.
The regular encoder cab is a RELATIVE speed control. There is NO position indicator nor feedback on the speed position since there are no stop and start points. The speed of the train is kept track by the command station only. Turning the knob up or down simply increase the current speed or decreases the current speed respectively. So if a given train speed is currently zero (Stopped), then turning up the knob will increase the speed above zero. Keep turning the knob and more increase in speed.
Likewise if a given train speed is NOT zero, turning the knob up or down simply increases the current speed or decreases the current speed respectively. Keep turning the knob up and more increase in speed. If instead you choose to turn down the knob, then the speed of the train will decrease.
The advantage of the normal encoder cab become very apparent when you switch trains.
Example: If you are running train A at half speed, and then select train B which is stopped, train B remain stopped while train A keeps running at is half speed. You can flip back and forth between two or more trains of different speeds and never have to worry about sudden speed changes. If you stopped a train you have selected, set the throttle down and unplug it but accidentally knock the throttle knob while it was unplugged, then plugging in the throttle will NOT result in the train suddenly moving.