It’s simulating gear ratios (1st gear, 2nd gear, etc.) using the hybrid transaxle by varying the engine speed relative to the axel with MG1. It’s a gimmick to make you think you’re shifting gears, when in reality there are no “gears” to shift.

 

manual shift
drive gear
1-4

 

Except there's 6 gears. Edited: Can't upshift manually in D.

 

D for Diarrheas

 

So it seems pointless to use the D D1, etc. No improvement in performance or mileage, right?

 

The main purpose that people have used the shifting for is to maintain speed going downhill. "Downshifting" will give you regen braking without having to use the brake pedal to maintain speed. However, downshift too much and the car may also add engine braking.

 

So what is the difference between D, D1, D2, D3, D4 and moving the shifter to the left and using S1, S2, S3, S4, S5, S6?

 

They are the same thing. You can use either to downshift the transmission.

However, the paddle shifters are temporary i.e. the transmission will go back to what it was before after awhile, while the S mode is permanent -- stays in the selected gear (unless there may be an over rev conditition): https://support.toyota.com/s/article/How-do-I-operate-the-7873?language=en_US

 

other than engine braking you can also "downshift" if you know you are going to accelerate hard in a second.

S means sport shifting which in general puts you more in control over which "gear" you are in. so it will hold that "gear" (to a point anyway) more so than using the flappy paddles in drive (D) mode.

 

D=driving, S=sport, R=racing 😀

 

It's all mostly for entertainment purposes. And fodder for internet discussions.