Exactly how does friction direction 2 work?

"Anisotropic friction" is just a fancy term for "directional friction".

Imagine a piece of velcro you're moving around. There is very little friction, UNLESS you are going in one specific direction. Right?

Alternately, imagine an ice skate. In a sense, there is little friction UNLESS you are trying to move exactly sideways.

So, set Friction Direction 2 to some direction - say "north".

Directional friction ("Anisotropic friction") is now turned on.

Simply, set DynamicFriction2 and StaticFriction2 to high values (say both .8) [footnote]

{Note - it is pointless setting FrictionDirection2, unless you also set DynamicFriction2 / StaticFriction2 to some high values, say .8.)

Set the ordinary DynamicFriction and StaticFriction to low values (say both .1).

{Note - in this demo, you won't see any difference unless there is a BIG DIFFERENCE between the normal and the "#2" values. So, set the normal friction values to .1 and the "#2" friction values to .8.)

So, the object will now be EASY to push in any direction .. but it will be HARD to push it "north".

To test this:

1. make a plane

2. make a cube

Set the cube exactly as I describe above. .1 for ordinary friction, and .8 for the "#2" directional friction.

Hit Play.

Now, click on the plane. Click the "E" key on your keyboard. You will now be able to tilt the plane.

You will now see easily that if you tilt the plane just a little, the object will begin to slide.

HOWEVER, if you tilt it "north", in the direction you set the "friction2", it will be very hard to make it slide that way.

indeed, get an image of a toy car and stick it on the cube. Make it so that the directional friction is SIDEWAYS relevant to the car.

the toy car will now feel remarkably "real" with that simple step. As you tilt the plane around, the car will "roll" forward and backwards easily, but it will be hard to make it slip sideways - just as a real car would behave.

So, that's all it is.

In a sense, a very simple explanation of direction2 friction is that often, for many objects in certain types of rigs, the direction2 is simply "SIDEWAYS" for the object - because many objects are harder to slide sideways

(You could actually name it 'sideways friction" rather then "directional friction" or "anisotropic friction" - since very often it is literally the sideways direction.)

So, imagine you're putting a toy Santa in a scene. he's easy to slide around in any direction, so there would be no anisotropic friction. But if you put a toy Santa's Sleigh in a scene, that is very hard to slide sideways, correct? So you'd add anisotropic friction in the sideways direction.

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[footnote] - recall that static friction is always higher than dynamic friction (I mean in both the real universe, and video game engineering). This applies to both the "normal" static/dynamic friction, and also static2/dynamic2 friction.