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Using brake and gas at the same time is called the "two footed" approach.

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. Above 1500 rpm the automatic would behave like a manual transmission. But going faster than 1000 rpm is not an option because you would be too fast for the coming rock. Hitting the rock hard might do some damage to tires 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. Bad! 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. Even more when you need to overcome an obstacle like this rock. More torque in low traction environments translates to spinning tires. Unnecessary trouble. So, you gave it a little 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 vehicle with automatic will speed up rapidly. The body will bounce and your rocker panel will end up on the rock. Damage is to be expected. Not good.

In order to avoid the stopping, to 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.

The foot on the gas will create more rpm and torque, the foot on the brake will slow the vehicle down.

Approaching the rock you would get on the brake
about 10 feet before and add some gas to maintain your slow crawl speed. The gas creates enough torque to climb the rock and the brake will keep you from going too fast. So, your speed is now almost as controlled as with a manual transmission. Slow before - slow up, over and down. No banging, no bouncing, no damage. That is why "two footed" is so valuable.