A.3. speed-drive: what's particular?
speed-drive increases gears.
This means: On a push of a button all existing gears are increased by a factor of 1.65.
Therefore, it's not just a "nice-to-have" gadget, that gives some additional gears - no - it offers a much wider gear range from very low to very high gears.
You can imagine the effect as follows: if you have a 30 tooth chainring installed, on a push of a button this small chainring acts like a 1.65 times bigger one, means a 50 t. chainring!
If you compare this range 30 -> 50 with a triple chainring, you can see, that you get a similar wide range as with three chainrings!