When you step on the gas, the stopper comes down on the leaf coming up. As a result, there are now two points at which the bracket is mounted, the forward point at the leaf spring pivot, and the point four inches back pushing down as the spring pushes up.

the spring doesn't just bend out of the way, because the chassis is trying to get it to come up (as a result of the leaf twisting due to the axle torque) so you end up with the downward force of the bracket canceling on the upward force of the spring. the front bolt essentially now has a rigid beam four inches(or whatever length they are) long to the back stop. The forces cancel(more or less) and you get a beam, to which the bracket is mounted, which in turn provides the revised IC.

Finally, while the front bolt is a pivot, the spring itself is not, it's bound by the front pivot as well as the rear shackle.
This is easier to visualise if you forget about the spring behind the axle, and think of a bar connecting from front eye to axle,leaving the axle free to rotate.Now you can see that the stopper can't just push on the front eye and pivot, it has to push on the tire and lift the chassis to do so.
so you can push down on the stopper and as a result push up on the whole chassis. Which is why the stopper on the cal track works.

From what I've found in discussion with others, the resultant IC should be at the intersection point of the Cal Tracs side view swing arm and the axle centerlines virtual arm.
Making it much like a four link in terms of IC projection.
Though unlike a four link, as the suspension moves, the IC won't move around like a four links would, but instead would move in relation to the motion of the front spring eye angle.

As a side note, based on this I see that you'd need springs that will deflect enough to work with these. Leaves with a forward spring pack would prevent the suspension from winding enough to engage the cal track. though you could always crank the preload way up I suppose.