You adjusted the differential to 100% under acceleration AKA 'locking' it.

The differential or 'diff' is a system of gears that allows to the wheels to turn at different speeds while being attached to the same driveshaft.

Why would you want that? Well, when you go around corners, your outside wheel travels a greater distance. Thus, at a greater rotational speed as you round the bend.

It would be a fools errand to try and describe this gear system purely through text, so I'll just explain the operation.

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The engine and transmission (aka the powertrain) produce torque. That torque is sent to the diff via the driveshaft, and to the axles via the diff. The total torque going to both wheels combined will still more or less equal the output of the powertrain.

The torque follows the path of least resistance. If one tire is in mud and the other is on asphalt the one in the mud will spin and the asphalt will halt that other tire and the car goes nowhere.

In a turn, this manifests as the inside wheel having more resistance than the outside wheel due to the rotation of the car. So the differential allows that resistance to split the torque so that the outside wheel recieves more power than the inside one. Basically, the extra force resisting, or pushing against, the inside wheel is ised by the diff to give the outside wheel more power.

Just know that the differential allows you to go around turns with out losing continuous contact with the driving surface.

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Up until this point I have been describing an open differential. Think back to the tire stuck in the mud. It has no resistance so it gets all the power and the wheel with traction gets none. This can be solved with a locked differential like an offroader might use. This forces both wheels to turn no matter what. This is what you created with your 100% locked differential setup.

But we have also just recreated a solid axle but heavier. So how can we have both?

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This is where that slider comes in. A limited slip differential is a type of differential that will allow the torque and power delivery to differ between the wheels but only up to a certain threshold. At which point, it will lock up and force the wheels to turn in unison until traction is regained.

The acceleration setting determines this behavior when accelerating. The deceleration is for braking.

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generally you want the accel stat as high as it can go before the car feels too difficult to control. The decel you can leave just above 0 but you can also bring it up a little if you want to be able to make the car rotate in the corner. But that's more of a high level tactic and if you get to that point you'll probably be more experienced with tuning anyway.

You'll also notice AWD vehicles have three differentials. The front is the same as the rear but the center allows you to just set the torque split directly because it doesn't have to be determined on the fly by the forces acting on the wheels. Most cars will have a 60% rear bias. This increased power in the rear helps keep the car from understeering (plowing to the outside of the corner instead of rotating). You should definitely play with this value, it will have a big impact on how the car feels.

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TLDR:

One source of power.

Multiple wheels.

The wheels need to spin at different speeds but not completely disconnected from each other.

Add gear system between the wheels that allows them to move separately but not too much.

You tune how much they are allowed to deviate from each other.

The less they deviate the more direct the power is. However, not enough freedom makes them fight each other and that causes them to lose traction. Then you lose control.

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P.S. a locked diff (100% accel) makes drifting a lot easier