Select whether to invert the backface culling (true) or not (false).

Unlike most other calls, this is not only related to stuff you draw via GL class. It changes culling of triangles globally. Major use case: rendering reflections for mirrors, water etc. Since virtual camera for rendering the reflection is mirrored, the culling order has to be inverted. You can see that the Water script in Pro Standard Assets does that.

	// Draws 2 yellow triangles and makes them appear/dissapear
	// depending on the vertex draw order
	// Notice that everything on the scene will suffer from the revert
	// backfacing as well
	var mat : Material;
	var swap : boolean = false;
	function Update() {
		if(Input.GetKeyDown(KeyCode.Space)) {
			if(swap) {
				swap = false;
			} else {
				swap = true;
			}
		}
	}
	function OnPostRender() {
		if (!mat) {
			Debug.LogError("Please Assign a material on the inspector");
			return;
		}
		GL.PushMatrix();
		mat.SetPass(0);
		GL.LoadOrtho();
		GL.Color(Color.yellow);
		GL.Begin(GL.TRIANGLES);
		GL.SetRevertBackfacing(swap);
		GL.Vertex3(0,0,0);
		GL.Vertex3(1,1,0);
		GL.Vertex3(0,1,0);
		GL.Vertex3(0,0,0);
		GL.Vertex3(1,1,0);
		GL.Vertex3(1,0,0);
		GL.End();
		GL.PopMatrix();
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Material mat;
    public bool swap = false;
    void Update() {
        if (Input.GetKeyDown(KeyCode.Space))
            if (swap)
                swap = false;
            else
                swap = true;
        
    }
    void OnPostRender() {
        if (!mat) {
            Debug.LogError("Please Assign a material on the inspector");
            return;
        }
        GL.PushMatrix();
        mat.SetPass(0);
        GL.LoadOrtho();
        GL.Color(Color.yellow);
        GL.Begin(GL.TRIANGLES);
        GL.SetRevertBackfacing(swap);
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(1, 1, 0);
        GL.Vertex3(0, 1, 0);
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(1, 1, 0);
        GL.Vertex3(1, 0, 0);
        GL.End();
        GL.PopMatrix();
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public mat as Material

	public swap as bool = false

	def Update() as void:
		if Input.GetKeyDown(KeyCode.Space):
			if swap:
				swap = false
			else:
				swap = true

	def OnPostRender() as void:
		if not mat:
			Debug.LogError('Please Assign a material on the inspector')
			return
		GL.PushMatrix()
		mat.SetPass(0)
		GL.LoadOrtho()
		GL.Color(Color.yellow)
		GL.Begin(GL.TRIANGLES)
		GL.SetRevertBackfacing(swap)
		GL.Vertex3(0, 0, 0)
		GL.Vertex3(1, 1, 0)
		GL.Vertex3(0, 1, 0)
		GL.Vertex3(0, 0, 0)
		GL.Vertex3(1, 1, 0)
		GL.Vertex3(1, 0, 0)
		GL.End()
		GL.PopMatrix()