What is the "two footed" approach when driving off-road?

Using brake and gas at the same time is called the "two footed" approach.
If you have a newer ML it will allow using the brake without losing traction control (ETS) - early ML will switch off traction control when you use the brake.

Here is why the two-footed approach is needed with automatics:

Lets say you are on a rock strewn trail. Rocks and boulders in your way
at all times - and no way to drive around them.
With a manual transmission in first gear low range you have a constant
low crawling speed and a constant uninterrupted power (torque) supply.
You are approaching a football sized rock with your front left tire at 1
mph. Hitting the rock with 1 mph is no task for the suspension, no big
deal for the tire, and no big deal for the available torque to move you
up and over at 1 mph. Going down on the other side of the rock happens
also at 1 mph. So, all in all a safe slow crawl over an obstacle.

With an automatic transmission and only the right foot on the gas you have a slow speed of 1 mph (around 1000 rpm) on level ground approaching the rock. Once you hit the rock your vehicle will come to a halt since the connection between engine and transmission is "fluid" - the torque converter is not delivering equal uninterrupted power below 1500 rpm. Going faster than 1500 rpm is not an option because you would be too fast for the coming rock. The ML320 moves at 4 mph with 1700 engine rpm in first gear low range. Hitting the rock with 4 mph might do some damage to tire and suspension - plus, the rocking and bouncing of the body, especially when coming down on the other side of the rock will slam the rocker panel into the rock. That will leave nasty dents. So you really need to go as slow as possible.

So, back to the rock. Your tire is still against it - engine still
running because other than with a manual transmission the engine will not die when the vehicle comes to a forced standstill. Now, to make it over that rock you will have to get on the gas to build up more torque. No big deal, there is plenty available. However, getting a vehicle moving from standstill
requires more torque then when already moving. More torque in low
traction environments translate to spinning tires. Even with ETS you don't want that to happen. Unnecessary trouble. So, you gave it more gas, the
tires did not start spinning and you are climbing up the rock. Nice and
slow. Good. But going down on the other side the ML will speed up
rapidly. The body will bounce and your rocker panel will end up on the
rock. Damage is to be expected.

In order to avoid the stopping, avoid spinning tires and the bouncing action you will have to drive with one foot on the gas and one
on the brake. Takes some practicing because our left foot is not that
sensitive on the brake. Approaching the rock you would get on the brake
10 feet before and add some gas to maintain 1 mph. The gas creates
enough torque to climb the rock and the brake will keep you from going
faster than 1 mph. So your speed is now almost as controlled as with a
manual transmission. 1 mph before - 1 mph up, over and down. No banging, no bouncing, no damage. That is why "two footed" is so valuable.

In order to keep ETS working for traction control the MB engineers had
to add the "two footed" feature.

Since the two footed technique puts more strain on the transmission
(unhealthy heat develops - and heat kills automatics) manual
transmission are still choice #1 when it comes to more difficult
off-road. For light and medium duty, and an occasional two footed climb
automatics are just perfect.
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