One-Touch Shift: How To
 

There is good info in the Manual like this on One-Touch Shifting:

Gear selector lever one-touch gearshifting With the gear selector lever in drive position D, you can limit or extend the gear range. Limiting gear range Briefly press the gear selector lever to the left in the D-direction. The transmission will shift to the next lower gear as permitted by the shift program. This action simultaneously limits the gear range of the transmission (page143).

Extending gear range Briefly press the gear selector lever to the right in the D+ direction.
The transmission will shift to the next higher gear as permitted by the shift program. This action simultaneously extends the gear range of the transmission.

Canceling gear range limit
Press and hold the gear selector lever in the D+ direction until D reappears in the multifunction display (page140). The transmission will shift from the current gear range directly to gear
Range D.

Shifting into optimal gear range
Press and hold the gear selector lever in the D- direction. The transmission will automatically
select the gear range suited for optimal acceleration and deceleration. This will involve shifting down one or more gears.

!Allow engine to warm up under low load use. Do not place full load on the engine until the
operating temperature has been reached. Shift into reverse gear R or park position P only
when the vehicle is stopped. Avoid spinning of a drive wheel when driving off on slippery road surfaces. This may cause serious damage to the drivetrain which is not covered by the Mercedes-Benz Limited Warranty.
Warning! G
On slippery road surfaces, never downshift in order to obtain braking action. This could result in drive wheel slip and reduced vehicle control. Your vehicle’s ABS will not prevent this type of loss of control.

iTo avoid overrevving the engine when down- shifting, the transmission will not shift to a lower
gear if the engine’s max. speed would be exceeded.

iIf you press on the accelerator when the engine has reached its rpm limit, the transmission will upshift beyond any gear range limit selected.