When I was rigging with Advanced skeleton in Maya, I found that ADV will match the bottom of the foot exact with XZ plane. I guess it is for the convenience of riggers who won’t place the big toe and the pinky toe joint exactly on the floor. However, from time to time we needs to rig a character whose foot is not exactly locate at Y=0 plane, maybe it has more than one pair of foots, or wearing a pair of high heel shoes. When it comes, the rock function would not work well on your character.
So this article helps you to solve this issue.
Let’s see the source code first:

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//Pinky/index toe side2side rocking
if (`objExists $toes` && `objExists $bigToe` && `objExists $pinkyToe`)
	{
	addAttr -k 1 -ln rock -at double -dv 0 ("IK"+$fitJointIK[$i]+$side);
	createNode -n ("IK"+$fitJointIK[$i]+"FootRockInnerPivot"+$side) -p $parent transform;
//			createNode -n ("IK"+$fitJointIK[$i]+"FootRockOuterPivot"+$side) -p $parent transform;
	setAttr ("IK"+$fitJointIK[$i]+"FootRockInnerPivot"+$side+".rotateOrder") 5;
	asAlign ("IK"+$fitJointIK[$i]+"FootRockInnerPivot"+$side) ("Roll"+$toes+$side) 1 1 0 0;
	$pos=`xform -q -ws -t $toes`;
	if (`objExists $bigToe`)
		$pos=`xform -q -ws -t $bigToe`;
	xform -ws -t ($pos[0]*$b) 0 $pos[2] ("IK"+$fitJointIK[$i]+"FootRockInnerPivot"+$side);
	parent ("IK"+$fitJointIK[$i]+"FootRockInnerPivot"+$side) ("Roll"+$toes+$side);
	duplicate -n ("IK"+$fitJointIK[$i]+"FootRockOuterPivot"+$side) ("IK"+$fitJointIK[$i]+"FootRockInnerPivot"+$side);
	$pos=`xform -q -ws -t $toes`;
	if (`objExists $pinkyToe`)
		$pos=`xform -q -ws -t $pinkyToe`;
	xform -ws -t ($pos[0]*$b) 0 $pos[2] ("IK"+$fitJointIK[$i]+"FootRockOuterPivot"+$side);
	parent ("IK"+$fitJointIK[$i]+"FootRockInnerPivot"+$side) ("IK"+$fitJointIK[$i]+"FootRockOuterPivot"+$side) $parent;

Looks complex, but we only need to pay attention to the code that clarifies the position of the controller. So here it is.

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...
xform -ws -t ($pos[0]*$b) 0 $pos[2] 
...
xform -ws -t ($pos[0]*$b) 0 $pos[2] 
...

You see it ignores the Y value and forces it to zero. That is why our foot are always gennerated on the ground.
Let’s modify it into this:

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...
xform -ws -t ($pos[0]*$b) $pos[1] $pos[2] 
...
xform -ws -t ($pos[0]*$b) $pos[1] $pos[2] 
...

We get its Y value back. Now restart ADV and try generate your ADV skeleton again, you can place your foot at any where you want and the rock function works well!