Wall against wall attack
Step one:
Look for a good spot where you set up the attack, a good spot has the following properties:
There needs to be a long straight piece of wall (the longer the better)
The straight segment needs to come right after one of corners of the defense
It is preferable if the side before the corner has a couple of turns in it
Step two:

At the right time, when the defender is more or less on the opposite side you grind the straight piece of wall in the opposite direction of the defender. You want to make sure you grind hard enough that you cant get between it when you're making a light grind. This step also gives you the necessary speed to at least equal the defenders speed. (if you aren't at least the defenders speed, abort and try again)
Step three:

At the end of the wall leave some room and head in the direction of the defender.
Step four:

Depending how much speed you got you can either harass the defender (and make him burn some rubber) a bit by following his tail just slightly in front of the end (enough so he can't get you with a tail-shrink attack) or go back on your own tail to get more speed. This kinda depends on the type of defense as well.
Step five:

When you get to the corner, you grind your own wall you left there in step one with sort of a light grind: you need to get between tail of the defender and the defender himself. For best effect you need to time this in such a way that you sort of hit the tail of the defender. If all goes well you will end up between the defenders end of the tail and the defender self. Because of the speed you get in the tunnel and because the attack happens right after a corner he usually won't be able to adjust quick enough to kill you.
The picture here is a bit misleading because the walls in arma don't have any width, it's basically four walls stacked against each other pretty tight.
Step six:

????
Step seven:

Profit!
Step five is the most crucial step, so you'll probably die quite a bit before you get the timing right. Not as hard as outgrinding someone with an outside cornergrind though.
The images are a bit rough, but i hope you get the picture. Questions and comments welcome.